Change to thermal view. Go down the ramp, head left and through the door. Pass down the corridor, through the second door and enter the control room. The lift (the thing with the big glass door) doesn't work right now, so go down the left hand side of it and smash the cupboard-type-thing and the grate behind it. Continue down the vent and go right to the end. Smash the grate you're standing on and kill the marine in the room. In one of the corners is a computer mainframe - smash it. Then turn around and destroy the grate in front of you, continue down the vent and smash the grate there, before again crushing the mainframe. The next grate is to the right of the mainframe, a little bit off the ground. Enter the vent, but halfway through it you should hear the beep-beep of a sentry gun and there's a turning off to the left. Go down that turning and you'll end up on grate overlooking a sentry gun. Destroy it and head back down the vent, before turning left to continue into the next mainframe room. The computer's on your left and the grate is on the opposite side of the room from it. Head straight down and enter the final mainframe room, destroying said computer. Go through the now unlocked door, and switch to electrical (red) view - it's bug time. Head left through the door, ignore the door on your left, then take the next left hand turning. Pass through the airlock and draw your pistol. Go down the corridor with the big 1 over it and open the door. You'll see an alien attacking a marine. Shoot the alien with your pistol. Now, continue through all the doors (there's loads), battling any aliens you meet, until you reach a big, long corridor. Depending on the difficulty level you're doing it on, there might be some field charge halfway. Continue down said corridor and enter yet another corridor. Head right, and follow the corridor until it turns, then take the first left. Go through the hole in the door, and go across the room to the other end where there's a grate. Smash it and enter the vent. At the various turnings keep these directions: right, left, left. Destroy the grate at the end and enter the "xenoborg-holding" room. Kill the facehugger in there (it might have been in the vent) and enter the door that was on the left when you entered. Go through the second door, kill the alien(s), then flick the switch. All doors are now unlocked. Head out through the two doors, then go across the room and exit it via the door. Head left now, and take the second right after the corridor turns. Go through the doors and enter a room "Where humans were holding xenomorphs." Go straight and get on the lift at the end, ride it up and continue down the corridor. Pass through the airlock, then go down the two sets of steps and follow the corridor round until you see a trail of Predator blood on the floor. Go straight on (don't follow it: stay on the same section of corridor) and take the first right (at the end of the corridor). Pass through the lab and the two doors at the other end, then kill the alien in the room and collect the security pass on the right hand side of the room. Go back out until you reach the main corridor again and now follow the blood trail left. Keep following it and eventually you'll reach a very large door which will open for you. Now follow the trail through many, many doors until you reach a large room with a big ball in the middle. If you have electrical view on you will see that there is a creature inside - you're fellow warrior. Select your Disc. There's a lever in one corner of the room - pull it to release your brother. However, releasing him reveals that the humans have turned him into a Predalien - use the disc to slay him. The base is now set to self-destruct, so it's time to exit. Head out through all those doors again (they're open) and when you reach the large room, go around to the left. The lift is still de-activated, so follow the wall round to the right and eventually you'll find a door. Behind that there's another door, and then there's a lever which activates it. Pull that lever and come back to the lt. Enter it turn around. When the lift reaches the top, exit the lift and you're in the control room at the beginning. Go through the door in front of you and follow the corridor round, then go through another door to enter the hangar. Enter your ship to end the level.