Paranoid Walkthrough:

MAP01:

Map Objectives: Proceed through map to exit, and enter Map02.
Most Direct Path: No explanation necessary as there are very few alternative paths.

1. Walk up the two sets of steps and choose your portal.
- Portal A gives you the mission briefing and puts you in the start of Map01.
- Portal B bypasses the mission briefing and puts you in the start of Map01.
- Portal C bypasses Map01 and puts you at the beginning of the action in Map02. This option is meant for players who have played through the game once and don't want to wander through Map01 just for the scenery.
The remainder of the Map01 walkthrough assumes you have chosen Portals A or B.

2. Walk through the doors past the lobby area. Explore some of the offices if you choose, but the main path will take you to an elevator.
3. Take the elevator down and through the various doors to the second elevator.
4. Ride the elevator up to Map02.

Map02:

Map Objectives: Pick up HEV Suit, pick up crowbar, find duct to Map 03.
Most Direct Path: No explanation necessary as there are very few alternative paths..

1. Walk past the Secure Access Door and notice the electrical control room to your right. Note the 3 fuse boxes, which will become relevant later in the game.
2. Note the vent grate and the locked door to your left.
3. You cannot access the main part of Map02 because the Barney tells you you need to put on your HEV suit.
4. Explore the office and the bathroom if you wish, then enter the HEV suit room. [This is where you will end up if you take Portal C in Map01.]
5. Press the keyboard to the only panel with the green HEV suit symbol. This raises the glass covering the HEV suit.
6. Grab the suit. Doing so "coincides" with the Resonance Cascade being triggered elsewhere in the Anomalous Materials Lab by Gordon Freeman.
7. Exit the HEV Suit room and retrace your steps. The Barney is dead, but the door has been shut. You cannot pass through without Access Override Authorization. This will become available later in the game. Pick up Barney's Beretta.
8. Head back to the start of the map and notice that your way is barred by a laser barrier. Don't walk through, as you will be incinerated. Instead, notice the duct whose grate cover has been blown up. Crouch and go through this duct.
9. You will come out into the room behind the locked door, and find the inside grate has fallen over. [IMPORTANT] You will need to crouch to get over this fallen grate cover. You will pick up a crowbar.
10. Find your way up to the control room. In front is a waste-water canal and catchment area. To the left is a ramp going down to the canal, but the door is locked. It will open later in the game. To the right is a locked door leading to the Fuel Room. This room is important later in the game.
11. To the right of the locked door is a small room with a set of stairs going up to a platform, and a crate. Pick up the radio on the crate for a message from your Controller.
12. Find the stairs, go up them, break the vent grate, and go through the duct until you drop down. This puts you in Map03.

Map03:

Map Objectives: Pick up Security Key, exit to Map04
Most Direct Path: Jump down from duct, walk East and jump to ledge/tunnel, jump down and continue walking East until first ramp. Turn left at ramp, enter Bay A3, go to lift at North-West corner of Bay, take lift up to control room, pass through and find stairs going up. Cross bridge across Bay A2 to a break room and take stairs down to security room. Security key is in alcove on wall. Return to break room and pass through to beginning of bridge. To right is gap in the floor and malfunctioning lift. Jump down to lift, then onto the ground, and head North to the bay doors to the outdoors. Jump up the crates to the outdoors, then head straight to the guard house and the locked door. "Swipe" (press your "use" key) the security key card on the control panel outside door, enter door, and turn right to exit map to Map04.

1. Break the vent cover and drop down. This is Bay A1.
2. Explore the area for goodies. Just to your right when you drop from the duct is a security room with a shotgun and ammo. To your left from the duct is a door that is locked. This returns you to Map02 later in the game after you have achieved other objectives.
3. The sliding bay doors to Bay A2 are jammed, so find another way around. At the East end of Bay A1 is a control room overlooking a tunnel of sorts. Jump up the crates to the tunnel, or go up to the control room and jump over the railings to the tunnel.
4. Jump down the far end of the tunnel. From here you can proceed to your left or straight ahead. Either path will work for now.
5. Assume you turn left, you'll reach a toilet. You can either turn left to Bay A2 or right to Bay A3.
6. Say you turn left, you'll be in a large storage bay with a truck trailer and an over-head bridge. There is a lift that is malfunctioning, and you'll notice a room overlooking the Bay. You need to get inside that room, but you can't do it from where you are.
7. From Bay A2 you can jump up onto the crates that are holding the Bay doors open to the outdoors, but for now return to the toilet and onwards to Bay A3.
8. Bay A3 is stacked with crates. If you head to the right (South) you'll come to a passageway that leads back to the tunnel to Bay A1. At the end of that passageway is the Hazardous Materials Bay A, which cannot be accessed right now because you do not have the Access Override Authorization.
9. Return to Bay A3, and explore the area. To your Southwest you can look down into the Hazardous Materials Bay A. Further to your West is a locked door leading to the Ordnance Storage Facility. Again it needs the Access Override Authorization. In that same area is a Maintenance Area and a set of stairs going up to a grated landing. The path to the stairs is blocked by a large crate. This area is important later in the game.
10. From Bay A3 you can go through the large bay doors in the North wall to the outdoors. If you turn West you'll come to the outside of Bay A2, a guard house, and a locked door that requires a security key. You cannot return to Bay A2 from here, so retrace your steps to Bay A3, pass it going East, and you'll come to a Loading Dock, a gate, a guard house, and a drainage tunnel. The door at the Loading Dock is locked, as is the door in the drainage tunnel. You will be able to open both these doors later in the game.
11. Return to Bay A3, and go to lift at North-West corner of Bay.
12. Take lift up to control room, pass through and find stairs going up. Cross bridge across Bay A2 to a break room and take stairs down to security room.
13. Security key is in alcove on wall. Press switch to lower glass and pick up key. Return to break room and pass through to beginning of bridge. To right is gap in the floor and malfunctioning lift. Jump down to lift, then onto the ground, and head North to the bay doors to the outdoors.
14. Jump up the crates to the outdoors, then head straight to the guard house and the locked door.
15. "Swipe" (press your "use" key) the security key card on the control panel outside door, enter door, and turn right to exit map to Map04.

Map04 & Map08: [NOTE: These two maps were originally a single map that was split into two. This walkthrough covers them together.]
Map Objectives: Find Standby generator, turn off electrified fence, open gate at fence, find Dr. Zimmerman, and obtain Access Override Authorization.
Most Direct Path: From the hallway that you enter go straight through the door and jump down to the waste water canal. Go through the door near the sign marked "Black Mesa Waste Processing Area 3". Take the stairs up and across the bridge over one of the areas. To the left is a door that takes you over a bridge to a Control Tower. Press the switch on the left, jump down from the bridge, and head for the large bay doors to the North. Go outdoors and turn West, going through the tunnel until you see a sign for a Loading Dock, and enter through the jammed doors. Go all the way South until you find the ramp going up, find your way into the Generator Control Room, and enter the room with the single generator. Press the Power switch to turn off the power and retrace your steps to the Control Tower. To the East of the Tower is a Storage Bay, through which is a doorway leading up to another Control Tower overlooking a fence and gate outdoors. Press the switch to open the gate, enter the door and turn into the first door to your right. Go through the room to the storage area behind, jump up the crates to the ledge, and break the vent grate. Go through duct to the other side, jump off the platform or take the ramp down, and take the stairs up to the other platform. Jump down to the electrical control room and shoot the remaining fuse that has not been blown. Go out through the door and proceed Eastward until you reach the elevator or stairs. Take either of them upstairs, continue Eastward, then down the stairs to the bottom floor of the prison. Find the door on the North wall, go through and open the door that is furthest West. Dr. Zimmerman will give you the Access Override Authorization. Return upstairs to find the door near the elevator locked. Pressing the switch triggers a booby trap and a part of the wall behind you explodes. Return towards the prison stairs and exit through the hole in the wall. Find your way through the caverns to the large outdoor pool. Follow the river to the drainage tunnel, take the door through, and find your way to the other door. This door brings you back to Map03 near the Loading Dock.

1. From the hallway that you enter you can either go straight through the door or you can jump up, break the vent grate, and go through the duct to enter the control room. Either path will bring you to the waste water canal. At the map start you will find a radio to get a message from your controller that tells you Dr. Zimmerman is being held in the Secure Holding Facility.
2. Go through the door near the sign marked "Black Mesa Waste Processing Area 3". Explore the Black Mesa Waste Processing Area 3, including the toilet and areas with storage vessels.
3. Take the stairs up and across the bridge over one of the areas. This will bring you to a hallway that has areas to explore. To the left is a door that takes you over a bridge to a control tower in a Storage Bay. To the right you head back towards the waste water canal, but there is a ramp that leads down and around, and eventually brings you to the bay with the control tower. You can explore the various areas, including the High Security Storage Facility, the office trailer, and the computer rooms.
4. You can either go outdoors through the main Bay Doors or through the office trailer. Instead, take the ramp up towards the office trailer, but turn left into the computer areas. Continue through until you come out into a control room with a door that leads to a water-treatment area.
5. At the North end of this water-treatment area is a set of Bay Doors leading to the outside, and at the South end is a ramp. Take the ramp and find your way into the Generator Control Room, and enter the room with the single generator.
6. Press the Power switch to turn off the power and retrace your steps to the Control Tower. To the East of the Tower is a Storage Bay, through which is a doorway leading up to another Control Tower overlooking a fence and gate outdoors.
7. Press the switch to open the gate, enter the door and notice the laser barrier blocking your way. On either side of the hallway is a door, but the door to the left is locked. Turn into the first door to your right, which is a security room.
8. Go through the room to the storage area behind, jump up the crates to the ledge, and break the vent grate.
9. Go through duct to the other side, jump off the platform or take the ramp down. You are now in the area behind the locked door.
10. Take the stairs up to the other platform. Jump down to the electrical control room and shoot the remaining fuse that has not been blown.
11. Go out through the door and proceed Eastward past the de-activated lasers until you reach the elevator or stairs.
12. Take either of them upstairs, continue Eastward [at which point you enter Map08], explore the prison control room, then head down the stairs to the bottom floor of the prison.
13. Find the door on the North wall, go through and open the door that is furthest West.
14. Dr. Zimmerman will give you the Access Override Authorization.
15. Return upstairs to find the door near the elevator locked. Pressing the switch triggers a booby trap and a part of the wall behind you explodes. Return towards the prison stairs and exit through the hole in the wall.
16. Find your way through the caverns to the large outdoor pool, follow the river to the drainage tunnel, and take the door through.
17. Explore the areas, and find your way to the other door, which is to the West of the waste treatment area. This door brings you back to Map03 near the Loading Dock.

Map03 (Return):

Map Objectives: Find a way back to Map01
Most Direct Path: Head South to Loading Dock, through doorway to Cafeteria, and up stairway to swinging doors that lead to control platform. Press switch to open main Bay Doors, jump down and run through the doors before they close. Head through tunnel to the area named Supply D-301, which has another set of Bay Doors with smaller door to left. Both are locked, so jump up the crates, shoot out the vent grate, jump into duct then into upper floor of storage bay. Take lift down, then take ramp up to adjacent storage bay, continue West up ramp, take right ramp, jump up crates to reach duct. [IMPORTANT] *Crouch* before jumping into duct. Drop down to enter Map01.

1. Head South to Loading Dock, through doorway to Cafeteria.
2. Exit the Cafeteria through the stairs to the South, and you'll find that the large crate has been moved away. You can either take the stairs up or re-enter Bay A3. [Note, you'll eventually need to get to the area at the top of the stairs, but you can get there through the Bay A3 path too.]
3. Before you take either path, remember to get the goodies from the Ordnance Storage Facility.
4. Taking the stairs will bring you to swinging doors that lead to a control platform.
5. But say you take the Bay A3 path, you'll head West and take the first ramp up to your left. Head toward the door near the sign Hazardous Materials Bay A, and enter. You can explore this area and take the lift up to Hazardous Materials Bay B.
6. At the East end of Hazardous Materials Bay B is the set of main Bay Doors.
7. At the North end of Hazardous Materials Bay B is the control platform you would have reached if you had taken the path in Step 4, above. To get there, jump up the grates.
8. Press switch to open the main Bay Doors, jump down, and run through the doors before they close.
9. Outside is a guard house and a tunnel. Head through the tunnel to the area named Supply D-301, which has another set of Bay Doors with a smaller door to left. Both are locked.
10. Jump up the crates, shoot out the vent grate, jump into duct then into the upper floor of Supply D-301.
11. Explore the upper floor, take the lift down, and explore the area.
12. Take the ramp up to the adjacent storage bay and explore the area.
13. Continue West up the ramp and explore the upper platform.
14. Find the duct vent on the North wall and jump up crates to reach duct. [IMPORTANT] *Crouch* before jumping into the duct.
15. Drop down to enter Map01.

Map01 (Return):

Map Objectives: Proceed through map to exit, and re-enter Map02.
Most Direct Path: No explanation necessary as there are very few alternative paths.

1. You will enter the map through a vent in the toilet.
2. Find your way through the map following the same path as the first time you passed through the map. Explore along the way.

Map02 (Return):

Map Objectives: Find gasoline can, re-start power generator, raise sluice gates to raise water level, swim across hole in floor, use crane to jump up to platform, exit to Map05.
Most Direct Path: Go through Secure Access Door and enter the electrical control room to your right. Shoot the remaining fuse that has not been blown, and deactivate the laser. Go through the deactivated laser barrier and continue to the doors to the Test Lab. Go through doors and head North. You'll eventually come to an area with a section of collapsed floor. Jump down and take the left fork along the waste-water canal, heading South. Keep going through until you come to the area with the control room overlooking it on the left. A door at the Southwest corner of this area has opened, so enter it and find the Fuel Room. Go up to the platform and find the gasoline can, return to the waste-water canal and head South and West along the canal to the water treatment area. Jump up the crates, break the vent grate, enter the duct and jump down into the transformer room, and press the Fuel switch. Press the switch to the generator, then head out to the switch near the windows. This will raise the sluice gates and the water level will rise. Return to the control room adjacent to the Fuel Room, go down the ramps and through the duct to the hallway near the deactivated laser barrier. Turn left and head towards the Test Lab and retrace your steps to the broken floor. Jump into water and swim to other side. [Note: This will take you directly towards Map05 and will cause you to bypass a secret area that allows you to get into Map06, the secret map. For directions to the secret map, read detailed walkthrough below.] Follow hallway all the way until you reach the elevator. Take the elevator down and enter the Test Lab. In the Northeast corner is crane next to a platform with a switch. Press the switch and quickly jump onto the crane while it is rising. Jump off the top to the platform above. Follow the ramp to the exit to Map05.

1. Go through Secure Access Door and enter the electrical control room to your right.
2. Shoot the remaining fuse that has not been blown, and deactivate the laser.
3. Go through the deactivated laser barrier. At this point you can choose to crawl through the duct to your left, and find your way to the Fuel Room. I'll assume you'll go straight instead, and it does not make much of a difference as you will end up in the Fuel Room eventually.
4. Go past the dead Barney and through the doors. You now have the Access Override Authorization, so you can pass through the doors. Explore the offices and the lunch room.
5. In the lunch room you'll see a door. This leads to Bay A1 of Map03, and now you can travel freely from Map02 to Map03 and back.
6. Follow the main hallway Northward and continue until you come to an area with a section of collapsed floor. You will see a computer room with its door on the other side (i.e., inaccessible right now). Jump down into the water canal below.
7. When you reach the main canal you can either turn left (South) or right (North). For now, turn right and notice the vent in the ceiling. It is too high to reach now. Turn around, and head South.
8. You'll jump down from a low sluice gate and pass a control room to the right before entering the catchment area you had seen during your first pass through Map02. At this point you have 3 choices - You can enter through the door to your right, you can enter through the door at the Southwest corner of this area that has opened, or continue straight through along the canal.
9. Enter the door that has opened in the Southwest corner and find the Fuel Room, the door to which is now open.
10. Enter the Fuel Room and explore it before going up to the platform and finding the gasoline can in an alcove in the Southern wall.
11. Return to the waste-water canal and head South and West along the canal to the water treatment area. There is a crate storage area at the furthest North end.
12. Jump up the crates, break the vent grate, enter the duct and jump down into the transformer room.
13. Find the generator in the adjacent room, press the Fuel switch, press the power switch to the generator, then head out to the switch near the windows. These windows are what you saw near the sluice gates in Step 8.
14. Press the control panel, which will raise the sluice gates, which in turn will cause the water level to rise.
15. Return to the control room adjacent to the Fuel Room, go down the ramps and through the duct to the hallway near the deactivated laser barrier. Turn left and head towards the Test Lab and retrace your steps to the broken floor. [Alternatively, you can go through the duct to Map03, drop into Bay A1, then go through the door to the left. This door has now been unlocked, and it will bring you to the lunch room in Map02.]
16. Once you reach the broken floor, jump down into the water but do not swim across. Instead, swim underwater to the ceiling vent you had seen in Step 7, above. Swim up and jump into the vent and crawl through the duct. This will take you to a secret area, but it will also open the door to the computer room outside (as seen in Step 6, above). You will need to go through the door to this secret room, and run into the computer room before the door to that room closes. You need to get into that computer room in order to access the secret level, Map06, and you have only one shot at it. [Save the game right before you make the dash into the secret room, so that you can reload the game and try again if you miss the first time around.]
17. Follow the main hallway all the way until you reach the elevator.
18. Take the elevator down and enter the Test Lab. In the Northeast corner is a crane next to a platform with a switch.
19. Press the switch and quickly jump onto the crane while it is rising. If you miss the jump, just wait until the crane has stopped, then press the switch again to make it come down, then try again.
20. Jump off the top of the crane to the platform above.
21. Follow the ramp to the exit to Map05.

Map05:

Map Objectives: Find the Research Labs, Recover Security Clearance, Exit to Map07
Most Direct Path: Exit the loading dock and go straight into the tunnel across the outdoor area. Take the doorway to the left, then down the elevator or stairs. Follow the hallway until you get to the large boiler room. At this point, if you have found the relevant secrets in Map02, you can jump up the crates to the platform and through the door to Map06. Otherwise, follow the hallway on the other side of the boiler room all the way until you reach a large open, sloped area. Keep going straight and up the ramp to a "turbine" area. At the other end of this area is another doorway. Follow it and go up the ramp to the rail tracks and go over the bridge across the canyon. Take the ramps down to the dam, jump down and enter the drainage pipe to the left, then find your way to the nuclear waste storage area. Find your way out, past the Research Labs, across the bridge over the warehouse, into the offices. The Security Officer's office is on the 3rd storey, and you need to enter the office to restore your security clearance. Return to the Research Labs and enter. Proceed until you reach the laser control room. Activate the laser, then return to find a door open in the corridor outside. Go through to Map07.

1. Exit the Loading Dock and explore the area. To the right is a guard house and gate, to the left is a fuel farm, straight ahead is the way into the main part of the map.
2. Go through the tunnel into the main part of the map. To the left is a door and straight ahead is another door, but after you enter it you find that your way is blocked by a loader. Return to the tunnel.
3. Go through the other door, and take either the stairs or the elevator down.
4. Follow the hallway to the large boiler room. As you enter the boiler room you'll notice a stack of crates to your left. If you entered the secret computer room in Map02 (near the broken floor) a crate would have raised to the point that you can jump up and onto the platform above. The final jump is a bit tricky and may require a few tries. There is a Secure Access door to the right that is malfunctioning, and another door to the left. Go through that door to enter the secret level, Map06.
5. If you do not enter the secret level, explore the boiler room and proceed up the ramp to the malfunctioning Secure Access door.
6. This will eventually lead to a large sloped area with pipes and two concrete block control rooms at the bottom. At the bottom is a doorway leading to a waste-water canal, which will take you to Map06. Unfortunately, this path is blocked before you can explore much of Map06. Return to the large sloped area and climb back up.
7. Explore the platform and go up the ramp to the "turbine" area. To the left is the hallway leading outside. You'll notice that the loader has been moved, allowing you to go freely back and forth through the door.
8. At the other end of the "turbine" area is another doorway. Follow it and go up the ramp to the rail tracks.
9. Go over the bridge across the canyon, with the dam to the left. You'll enter a room with a door to the right that cannot be opened.
10. Take the ramp down and enter a storage area that leads to the dam. To the right is a snack room, which in turn leads down a series of ramps to a large warehouse. Explore that area if you wish, or continue to the dam instead.
11. When you reach the dam jump down and go under the bridge until you come to a large drainage pipe with one of its bars broken. You can explore further if you wish, but you'll need to return here.
12. Climb into the pipe and find your way to the nuclear waste storage area.
13. Take the corridor to the Research Labs, but you'll find that your security clearance has been revoked. Notice that the door you couldn't pass through in 9, above, is now propped open with a crate, allowing you to pass back and forth that way.
14. Continue past the Research Labs and enter another storage area, then onto a bridge over the warehouse you may have explored in 10, above.
15. You'll enter a 4-storey office building, which you can explore for goodies and fights. You can even go outdoors, and explore the warehouse if you haven't done so already.
16. On the 3rd storey you'll find the Security Officer's (Sgt. Harrison) office. He will restore your security clearance, and give you access to the adjacent storage room with ammo, armor, and health.
17. Return to the Research Labs and enter.
18. From here on, it's quite linear and there are no significant branching paths. Destroy the various alien test specimens when you encounter them, and continue until you reach the laser control room. Just before you reach that room you'll pass a locked door to your left. After you've activated the laser to kill off the alien specimens, return to this door, which is now open. It will lead you to Map07.

Map06:

Map Objectives: Proceed through map to return to Map05
Most Direct Path: Go through control room to waste-water canal, and follow it until you come to large open area with really deep water. Take the tunnel to your right and follow the canal until you come to a flow control area with two concrete block control rooms. Exit through the door to the Supply B-301 storage area. Go outdoors through the main bay doors and go through the tunnel to the Hazardous Materials Bay B. Go through the Guard Booth and press the control panel. The door only opens partially, so crawl under before it shuts. Go through the HazMat Bay B to the doorway in the back until you come to another control room. Press the switch to raise the gate in the water, run outside, and swim under the gate before it shuts. Swim to the control room on the other side, enter it, and go through the door in the back. You're back at the canal, and across the canal a door has opened. Go through and take the stairs up to a catchment area with a canal. Take the canal until you drop off, then take the right fork to return to Map05.

1. Go through the control room to the waste-water canal, and follow it until you come to a large open area with really deep water. There are three other tunnels that pour into this deep pool - the one to the left is barred and leads to Map05 (you'll reach it later in this map), the one straight ahead leads to two doors that are both locked at this point, and the one to the right leads to the rest of the map.
2. You can either take the tunnel to the right, or you can jump into the pool and swim all the way to the bottom. In the Northeast corner is an underwater tunnel that leads to a secret area. This area eventually brings you to the rest of the map.
3. Assume you take the tunnel to the right instead of jumping into the pool. It will take you to a flow control area with two concrete block control rooms. Explore the area before exiting through the door.
4. You are now in a storage area, Supply B-301. Explore the area, including taking the lift up to the upper floor. There is a control room that is locked. This is the area that you would eventually have reached if you'd taken the underwater path through the deep pool. On the other side is a door that connects to another storage area but it is blocked by crates.
5. Exit Supply B-301 through the main bay doors and go through the tunnel until you come to the Hazardous Materials Bay B. There is a guard house to your right, a water treatment area straight ahead, and the bay doors and a guard booth to the left. Explore the area before entering the guard booth.
6. Press the control panel. The door only opens partially, so crawl under before it shuts. You'll be in a security room.
7. In the security room is a door that leads to the Hazardous Materials Bay B. Explore the Hazmat Bay, finding the corridor that leads back to Supply B-301. By now a crate has been removed, allowing you to move freely back and forth between these areas.
8. From the HazMat Bay, take the doorway all the way at the back until you come to another control room.
9. Press the switch to raise the gate in the water, run outside, and swim under the gate before it shuts.
10. Swim to the control room on the other side, enter it, and go through the door in the back. You're back at the canal, and across the canal a door has opened. (These were the locked doors in the tunnel across from the deep pool.) If you follow the canal it will take you back to the deep pool.
11. Go through the opened door and take the stairs up to a catchment area with a canal.
12. Follow the canal until you drop off. If you take the left fork you'll come to the barred tunnel overlooking the deep pool; if you take the right fork you'll return to Map05.

MAP07:

Map Objectives: Find the Data Control Operations center and upload data virus, make your way to the pick-up point to exit.
Most Direct Path: Pass through door and take corridor to elevator. Ride elevator down and proceed to control room above Test Lab. Press the button until the Gargantua teleports away, enter the Test Lab to pick up the security key, then return to elevator. Ride elevator up, go outdoors, and destroy Gargantua. Proceed through door on the other side and make your way down to the guard booth outside the Data Control Operations center. Go through the door to the maintenance area, take the ladder up and crawl through ducts. Drop into an office, proceed to Data Upload room, upload virus, then exit through doors until you reach the guard booth. Use switch in guard room to open door, go through other door that's opened, and take elevator up. Proceed through door to canyon, which leads to a cave, which takes you to an outdoor area for the final battle.

1. Enter through the door, and explore the storage bay to your left, and the locker room & toilet ahead to your right.
2. Around the corner is a double set of doors to an elevator. If you continue along the corridor instead of taking the elevator, you'll see a weapons locker to your right and a door to an outside rail yard. At the far end of the open area is a door (marked Lambda Complex) whose switch requires a security key card. You'll need to go through that door, but for now, return to the double doors and take the elevator down.
3. Follow the corridor to a control room and a door to the Test Lab. You'll get a message about your access being denied because live specimens are present in the Lab.
4. Continue up the ramps and corridors until you come to the control room for the Test Lab. You'll see some angry head crabs and a security guard attacking a Gargantua in the lab. The control room has a switch, which you should press to generate a high-voltage current that zaps the Gargantua. You'll need to keep pressing the switch, and after the fourth time the beast teleports away.
5. Return to the Test Lab doors, and this time you'll find that access has been granted. Proceed through the door at the far end of the Test Lab and enter the security office to your right; press the switch to get the key in the alcove. Proceed up the ramp and explore the area or return to the elevator through the Test Lab, as either way you'll be coming back to the elevator.
6. Assume you go towards the ramp, you'll be in a corridor that leads to a door that is malfunctioning. Through the partially opening door you'll see an outdoor area with a helipad. You'll eventually find your way there.
7. Take the ramp up to an outdoor fuels storage area with a fence that overlooks a part of the open area you saw in 5, above. At the far end is a door.
8. Go through the door to find yourself in a double-storey storage room, with an open duct on one wall and a lift that takes you to the lower level. Take the lift down if you wish, but you'll find the door down there locked.
9. Crawl through the duct. You'll find yourself in a space right above an instrumentation room you had passed before, in 3, above.
10. Follow the corridor back to the elevator, and ride it back up. Turn left and follow the corridor to the outdoor rail yard.
11. Kill the Gargantua (yes, this is the same one you were zapping in the Test Lab, and man! he's pissed). Use the switch to the door at the far end, and enter the Lambda Complex.
12. Proceed down and through the areas until you come to the offices. To the left are 2 doors, to the right is one door, straight ahead is a guard booth and the door to the Data Control Operations center (which is locked, with the switch inside the guard booth), and another locked door.
13. Return to one of the doors you saw to your left, and go through into a maintenance area.
14. Take the ladder up and crawl through the ducts. You'll come to a spot in the duct where the grate has fallen off, allowing you to jump down. Don't jump down, but jump across instead and follow the duct until you reach another fallen grate. Jump across and follow the duct to pick up some goodies, return, and jump down the opening. You'll be in a room that you would otherwise have been unable to reach from outside it (considered a secret). Use the HEV charger then jump up over the crates and explore the rest of the area.
15. At the far end you'll reach doors that take you to a corridor beyond. However, the way is blocked by crates, and you have no option but to return.
16. Find your way to the center of this area, which has the Data Upload station. Upload your virus, then return to the doors.
17. A grunt will blow up the crates, but by now the corridor is flooded with nerve gas. Run through until you reach the guard booth you saw in 12, above.
18. Use the Fume Hood switch to evacuate the nerve gas if you wish, and use the other switch to open the main door. As you pass this door, the adjacent locked door will open.
19. Take the elevator up. To your left is the now-open door to the storage area in 8, above. To your right is a door to a canyon.
20. Follow the canyon into a tunnel, pass the cave and into the tunnel beyond, until you jump down into the open area you saw in 6, above. [TIP: Instead of jumping all the way down, try to land on the narrow wall below, and follow it to the crate with goodies on top. This is a secret area that can only be reached from the wall.]
21. Kill the Gargantua to trigger the end cut-scene and complete the game.