After the cutscene avoid those rats that will come out of the valve. Head in and another valve with rats will come out. Aren't they annoying? Head left then right. Climb up the ladder and head right. A Fester will be here so have your Molotov's ready. Grab the stuff on the left and the Flares in the locker. Destroy the wood boards on the right wall to find some TNT. Head upstairs to the Supply Room. Two Festers will be down the hallway. Go back downstairs and get some more Molotov's. Inside the Supply Room you'll find some Mainliners and a Burrower. Grab the ammo on the desk and get the stuff on the shelves in the corners. Go to the next area and find some Xombiums behind the door. Kill some more Mainliners in the Munitions Room and get the Xombium and Flashlight Batteries on the desk by the door. Grab more Xombiums by the cell doors and head over to the 3 wheels on the wall on the left side of the room. Keep turning the wheels until you can get over to the other side. Grab the stuff on the shelf then go under the stairs and cut the boards up to find some more stuff. A Nooseman will be around here too. Head upstairs, get the stuff on shelves and lockers then enter the Medical Lab. Don't kill the caged Slayers. Grab the Xombiums in the right corner on the sink and lower the lever to open the door. Go right for another flashback followed by some Slayers and Festers. Head in and now your back at the Death House Basement. Head back to the beginning of this area (where you fought the first Marksmen, remember?) and the dead dude will open the door for you, to the right of the dead CO. Go in and head right to enter the blue door at the end.