Objective: Exploration
After arriving on the Alien ship, work your way through the dark tunnel till you drop into a green-tiled corridor. First head north and drop into the big room there. Go through the east door and down the steps to the room containing switch 1. Flip it to open door A, which you'll reach later. Go back to large room and take the elevator to the south to the corridor above. Work your way around the tunnel, until you return to the familiar green-tiled corridor. This time head south. In the following room are two mechanical sentries, with considerable firepower. Your best tactic is to keep moving until you've finished them off. Head to the far wall and flip switch 2 to lower a platform behind you, which will take you to a room above. You'll find a corridor leading right to switch 3, which you must flip. Return to the lower room and exit it by using the concealed platform in the center (the one with the alien gun sitting on it). Eventually you will come to a large oval room, with a bridge allowing you to cross the lava pool. Shoot switch 4 (possible with grenades, but easier with shotgun or plasma rifle), then go through door D. now turn right and head to the small room ahead. From there you can return to the satellite to save and recharge.
Once you're back on the Alien ship, leave the small room and follow the rest of the corridor till you reach switch 5. Flip it to open door E. Afterwards return to the large oval room through door D, ride the platform at the other end of the bridge to the corridor above, and find door E. Once you've passed it, head around to the door in the south-east wall. Follow the corridor behind it, turning left to door A. Flip switch 6 beyond it. Then head south until you drop down into a dark lower tunnel. To the north is a lift which will return you to one of the original tunnels leading back to your entry point. If you wish, you can head south, returning to the satellite beam-out point; otherwise, head north back to the large room there. Go through door F. Now simply head south until you reach the end room from where you can see the satellite. Approach the window and you will be teleported back to the satellite on the next level, Savage Streak...
