This act begins after the Junker dies. Marcus and crew must continue on foot. Start following the path. As you near the shack some light will come on. When you actually get to the shack the lights go out again.
Continue down the path and when Marcus gets near the right turn some glowing wretches will run down the pipes.
Go to right at the right turn. Keep following this path to the door. A cut scene will begin. After the cut scene the squad will split in to two groups. Marcus and Dom will go down a path to the right.
Follow this until you go through a gate. In the area after the gate there is another gate to the left. Go through it and prepare to meet the new wretches. Take cover behind the rock right in front of you.
A short time after these wretches die they pop, which will hurt you. When the wretches are dead go through the open doorway they came out of. Follow the catwalk to the destroyed section. Take the left path.
Follow what look like tracks in this area. Enter the door thatís ahead and to the right. Walk up to the desk on the right. There is a button here, push it.
After you push the button and exit this room, you will see a room back the way you came from. Head over to that room. On the way there a wretch will attack.
Enter the room and walk on to the elevator. Push the button. Marcus and Dom will ride the elevator to the roof.
On the roof there is a path to the right. At the bottom of the tin ramp, turn and head left to the open window. Jump in through the window.
Straight across from the window is a door to a hallway. Exit this room in to the hallway. A conversation with control will begin.