1. Campbell is seen hunting for Sarge in the opening cutscene.
2. This Imp charges through the shattered window on the left. Another Imp spawns behind you. Grab the armor on the right before climbing through the window on the left.
3. Bullets and a shotgun are on the right. The cabinet on the left can be opened with code 571. Inside is a rocket launcher, rockets, and grenades. Update: Stu G. writes, "Screen Shot 3 - showing ammo locker (access code 571) - to its right is a bank of monitors. The extreme right monitor is a touch screen, controlling the opening of a security locker containing security armor that is located just to the left of the health terminal shown in Screen Shot 4."
4. This Wraith attacks as you leave the room. Note the health terminal on the left.
5. Keep left and battle a swarm of Cherubs. An Imp emerges from beneath the first room on the left. An ammo belt, clip and cells are in the second room on the left.
6. Follow the corridor and a Cherub-summoning Archvile spawns. Rush him with your chainsaw and he'll go down quickly. The chainsaw also dispatches any Cherubs he's summoned. Approach the door ahead and your objectives are updated -- obtain the access codes for Lab A.
7. Backtrack past your initial position then pop this Imp with your shotgun. A health station, armor and shells can be found in the office behind him.
8. Approach the door to the left and receive this new objective.
9. Press on through this unlocked door.
10. Carefully step across to the door ahead. A voice warns, "Sarge is waiting for you."
11. Imps and Trites attack beyond this door.
12. Be careful when using this security scanner on the right. If you select "Main Entrance Hall" a Trite will attack from behind the screen.
13. A Commando attacks from the hallway. Defeat him and this second Commando spawns. Note the armor on the floor.
14. Grab the bullets, cells and shells on the right before approaching the desk ahead.
15. Use this panel to open the storage area.
16. Crawl through the open panel here.
17. Inside you'll find shells and Tony Bates' PDA, alongside what is left of Mr. Bates. Listen to audio log, "Lab A Security Door" to learn the code 627.
18. Return to the Sector A door, battling Trites along the way. Enter code 627 to unlock the door.
19. Enter and the corridor transforms itself.
20. Carefully hop along to the opening ahead.
21. Through another door and this Cherub attacks from a secret compartment. More follow.
22. This armor is a trap. Approach it and a secret panel slides open and an Imp attacks.
23. A few steps farther and this Wraith and another Imp attack. This is close quarters so I'd suggest the chainsaw.
24. Approach the supplies on the right and another Wraith and more Cherubs attack. Heal at the health station. On the left is Charlie Haskell's PDA with the code 468 to locker 669. Continue through the door on the right.
25. Hop up to the supplies on the right. You'll find rockets, armor and an ammo belt.
26. Don't bother trying to make this impossible jump.
27. Instead, backtrack past more Commandos and Cherubs.
28. Ride this lift up to level 2.
29. A Wraith attacks as you step off the elevator. An Archvile spawns that summons Cherubs. The Soul Cube takes out the Archvile quickly.
30. Ready your chainsaw as you continue into the room. Wraiths attack, followed by Trites.
31. In large numbers the Trites are particularly nasty. Back up to a wall so they can't get you from behind, then decimate them with your chainsaw as they approach.
32. Search the area for supplies, including shells and these grenades.
33. A medkit is to the left. Shells and a clip are to the right.
34. Don't miss the armor vest behind this crate.
35. Ride this lift up to level 3.
36. Cherubs and an Imp attack as you exit the lift. Use your Soul Cube to recover health if you need to.
37. A Commando attacks through the door ahead. Armor is to the right. Clips, shells and an Imp are to the left. Continue through the door ahead.
38. Imps attack from either side as you step down the corridor. A Commando is straight ahead.
39. Shells, an ammo belt and cells are to the right. Ready your plasma gun before picking up the armor on the floor.
40. Two Imps attack once you pick up the armor, followed by Cherubs.
41. Continue through another door and this Imp attacks.
42. Open cabinet 669 with code 468. Armor, grenades and cells are inside.
43. Ready your plasma gun before retrieving the replacement door panel on the right. An Imp and Trites immediately attack.
44. Start backtracking to the lift. An Archvile and Cherubs attempt to block your path.
45. Ride the lift back down to level 2.
46. A large herd of Wraiths attacks as you exit the lift.
47. Return to the other lift and ride it back down to level 1.
48. Keep left off the lift and backtrack across these panels.
49. Return to this door and replace the missing panel. Continue through the door.
50. A medkit and a Trite are through the door on the left. These two Imps are waiting through the door on the right.
51. Ride this lift down to sub-level 1, Main CPU.
52. Campbell warns you with his dying breath that Sarge has his gun, the BFG. Proceed to the bay door ahead. Update: Frank writes, "To the left you just see a hole in the ground ... drop in and crawl to the other side to find a stash of goodies to reload yourself."
53. Activate the panel to finish the level.