After the conversation, go down the hallway to the left. At the end of the hallway will be a button. Push the button. It will break.
Marcus will then say he needs to find an alternative route. Go back to the room you entered building from and take the route to the right. At the end of this hallway is also a button.
Push this button and the door will open.
Enter the hallway behind this newly opened door. About half way down will be a door to the right. Walk down this hallway to the door and kick it open.
When the door is opened a cut scene will begin. Enter the room through the door. Go to the left to a door with a button. When Marcus opens the door a stranded will shoot at Marcus.
Marcus tells him to help open the other door.
Exit this room and continue to go to the other side of this room. There will be another door. The stranded will open the door. Follow the walkway around this room. Go through the door at the other end of the walkway.
Go straight ahead until you get to the room with the pillar. Take the door that leads to the right. In the next room the stranded will open a door to a big room.
In the big room, go to the left. At the far end of the walkway the path will bend around and head back the way you came from. Follow this walkway to a door at the other end. Enter the room through the door.
In the room go left and exit this room through the door. Go down the steps and through the door to the right.
In this room some of the planks are rotted to the core. They will break if Marcus walks on them. When you enter the room, go to the right. Just as the area gets bigger, go left. Stay out of the middle of this section.
Move forward until you get to the opening between machines on the left. At this point cross over the planks to the wall on the right.
Once at the wall follow the wall to the left to the corner.
At the corner follow the wall to the left until Marcus reaches a door to the right. Enter the door.
Cross this room to the door with a button next to it. Push the button.
Enter the small hallway and then the big room after the hallway. On the right side will be a door with a button next to it. Push this button and enter the next room.
Start across the room to the far side. About half way across Marcus will be attacked by wretches.
Finish crossing this room. Enter the small room through the door and continue right in to the hallway beyond the small room. Turn right and follow this hallway.
Just before the room at the end, a wretched will attack. Kill it, then enter the room with a chair. Exit this room through the door to the right.
There are two doors from this room. You need to turn the primer valve before they will open. The door to the right of the valve has some ammo and a hand gun. The door to the left has some stairs.
Take the stairs down. Follow hallway until Marcus reaches a door to the right. Go through the door.
Head to the left side of room and there will be a door there. When Marcus reaches the door Baird and Cole enter the room and regroup with Marcus and Dom. Marcus must turn the wheel to open these gates. Turn the wheel.
Enter the large room through either gate. Someone will notice a wretched on the ceiling. Bring its world crashing down. Then a bunch of wretched and grub Locust will attack.
After you kill them, head down a hallway to the other section of this room.
There may be some Locust over here as well, if they didn't join the other locust in fighting you. In the next section turn and head left to the end of the room. To your right will be a door. Kick the door open.
The door won't open. Marcus will tell Jack to open the door. While Jack is opening the door, a Boomer and a grub will come down an elevator.
Kill them and then wait for Jack to finish opening the door. When it is open enter the room through it.