1. Talk to the chap ahead who will start your new squad. Bust open the supply crates then ready your rocket launcher before exiting to the street outside.

2. A second squad mate will join you in the street. Move fast or else he'll be sniped. Train your laser site on this window. (The blue laser light is coming from the sniper.) Your rocket will take the sniper out in a single shot.

3. Behind you is a Combine crusher. Duck under it to the right and find a fallen comrade with supplies, including a medkit, crossbow and pulse-rifle secondary ammo.

4. A second sniper is down this alley on the right. If you're careful, you can line up your site through this truck window and be safe. Your squad mates, however, are still vulnerable. A third squad mate and a medical supply crate are in the storefront on the right. Grenades are next to a body on the other side of the truck.

5. Enter the head crab-infested warehouse through this door.

6. In addition to the head crabs, beware this toxic zombie. Ammo and medkits are in a bathroom on the left. Continue down the ramp in the next room.

7. A wounded comrade is below, as well as more zombies and supply crates.

8. Continue through this door after defeating toxic zombies.

9. Zombies await at the top of the stairs. Energy is in the crate and a trunk full of grenades is beyond.

10. A sniper is across the street. Toss a grenade to his window after he's helped kill off the zombies.

11. Here's the trunk with infinite grenades. Barney can be seen upper-right. He tells you to use grenades to clear out the snipers pinning him down. Continue up the stairs to the right.

12. There are two snipers you must now take out. The first is here to your left.

13. The second sniper is straight ahead. I actually found it a bit easier to drop to the level below to take out this guy.

14. Follow Barney to the street then quickly disable these hoppers while Barney opens the nearby gate.

15. Take the medkit from this crate then enter the doorway ahead. Follow the hallway and kill a couple toxic head crabs to load the next zone.

16. Join up with a medic and enter this courtyard. Toss some flammable barrels to topple an automated turret and defeat the Overwatch that come down these stairs. Retrieve grenades from a supply crate on the left then continue up the stairs and through a door where you'll find even more supply crates.

17. Climb the stairs and look out the window to what Barney calls the "Overwatch Nexus" across the street. Listen to Dr. Breen plead for your surrender as Barney unlocks a door to the left.

18. Your squad destroys this automated turret, but you've been spotted. Rush down the stairs (to my left in this shot) to escape an Overwatch barrage.

19. Step outside to witness this heavenly beam of destruction - the aptly-named suppressor.

20. Rush to the dumpster seen behind Barney for medkits. Keep moving to avoid the deadly beams.

21. This is what happens for those who get caught in the suppressor beam. Sizzlean.

22. Sprint along the left side of the road then launch an assault through this door. Toss grenades inside while your squad fires at the Overwatch.

23. Heal up at the health station then exit to this plaza. Avoid the suppressor beams while sprinting to the small bunker ahead, where an Overwatch awaits your pulse rifle.

24. Once in the bunker, you are safe from the suppressor fire. Wait for your Aux Power to recharge then stage an assault through this door to rescue captured comrades.

25. Shoot a padlock to free this comrade on the right, grab the supplies on the left, then exit through the door on the right after readying your pulse rifle.

26. Charge up the stairs and engage the Overwatch guarding the Nexus. This is an excellent time to use your pulse rifle's secondary fire.

27. This shot illustrates how the suppressor works. The beam originates from the turret on the Nexus' roof, is deflected off something in the sky (presumably an orbiting satellite) and then back down to the target.

28. Find this force field at the far end of the courtyard. Use your gravity gun's secondary-fire to pull the plug. Use the medical station on the right then proceed up the stairs to the left. You are now inside the Overwatch Nexus!

29. Once atop the stairs, enter this room through double doors on the left. The room seen here on the right holds supply crates and an infinite-ammo grenade trunk. Note the blue lasers ahead, which trigger an automated turret.

30. Step into the blue laser light to trigger the turret, then back up to get out range and toss a grenade. Once the turret is destroyed, Barney unlocks the door ahead with the console on the right.

31. Bust this supply crate for medkits and defeat the overwatch ahead. Repeat the grenade-in-the-turret tactic on a pair of turrets on the right. Return to the trunk for more grenades as needed.

32. Fight your way through the room then up a flight of stairs to supplies and more comrades-in-arms. Barney deactivates the force field and they join your squad.

33. Direct your squad mates to stay in the room while you destroy these two turrets with grenades. Disable the generator ahead with your gravity gun like you did in the previous level, and Barney informs you that there are two more generators to go.

34. Use the medical and energy stations on the right and Combine troops attack. More supplies are under a table far-right. You can go left or right through the doorway ahead -- let's go left.

35. Keep right then enter this room. Approach the doorway ahead and the Combine block it, along with the door behind you. While your troops engage the Combine through holes in the walls, place the hoppers seen on the left in front of the two doors. Clean out the Combine once they break in then use your gravity gun to access the supply crates upper-right. Continue through the door ahead.

36. Turn left and grenade these two turrets. Make sure you hold down primary-fire for a bit to throw the grenades with some extra force.

37. Enter this room on the left to find the second generator. Pop it with your gravity gun then grab the supplies and fill up at the health station. Direct your squad to leave the room first to avoid certain death.

38. Backtrack past the previous door, this time going right. The desk ahead helps with this turret. Pull it to you then use it as a shield while you approach and toss a grenade at the turret. Bust the supply crates for medkits, pulse-rifle secondary-fire ammo and grenades.

39. A tricky jumping puzzle awaits you in this room on the right. Deactivate the hopper then jump from (1) to (2) to (3).

40. Continue the jump-by-numbers, then disable several hoppers on the other side of (3).

41. Duck under this next laser then turn right.

42. Jump to (1), careful to avoid the laser before jumping to (2).

43. Follow the arrows to climb the pipe and drop over the other side.

44. Press the indicated button to deactivate the lasers. Grab supplies while your team joins you.

45. Stay right to find the third generator. Zap it with your gravity gun.

46. Follow Barney to this large firefight in the lobby. Your crossbow and the secondary fire from your pulse rifle work well here. Once the area is cleaned out, proceed under the "Exit" sign ahead.

47. The only thing more annoying than your squad mates constantly telling you to reload, is this sequence where Barney is constantly telling you to follow him to the roof, while you are busy disarming the hoppers he keeps charging into. Fill up on health and energy at the stations on the right and grab supplies from the crates on the left. Also don't forget to pick up any ammo dropped by the Overwatch.

48. Proceed through the door on the right that Barney opens for you, then fight your way up the stairs to the roof.