C3: When you exit the elevator, you will arrive in a room with two napping dogs. Ignore them and exit through the southern door, which requires card 5. If you found the antenna, Schneider will be nice enough to give you a call on the transceiver and warn you that the basement has a lot of poisonous gas. Prepare card 5, an open the upper left door and inside, you'll find a prisoner. Then use card 5 to open the left door. Before you pass through it, activate your gas mask, and then proceed to room C2. Alternately, you can use card 3 to open the door in the upper right to access the basement maze from a different direction, but ultimately, you will be forced to backtrack since you won't have the right access cards to proceed.
C2: From C3, you will have no choice but to follow the path north to B2. If you've arrived from the left side of B2, you can follow the U-turn in the path and return north, but this will ultimately lead to a dead-end. This room will be of interest to you in the future however.
B2: No matter where you came from, all you can do is follow the paths from one location to another. Following the walkthrough, you will proceed east to B3.
B3: When you arrive here from B2, it may appear that you've stumbled upon a dead-end. But it never hurts to inspect the walls. If you punch the wall in the upper right corner, you will detect a weak-point. Place a plastic explosive on the wall to blast it open, and proceed north to A3.
A3: From the left portion of B3, you will arrive right in front of a door. This door can be opened with access card 1, and you'll find a prisoner. If you've followed the walkthrough to this point and rescued every prisoner, you will be advanced to the final class of four stars. This is a special class, as it is needed to access an important item later on, so it is crucial that you do not lose it by shooting any prisoners. This prisoner will tell you that the only way off the floor is via the north elevator, which makes sense considering how the south elevator won't go up. Open the door, put on your gas mask, exit the room, and follow the path west to A2.
A2: You will arrive in front of another door. This one requires card 5. When you step inside, it will appear that at long last, you have found Dr. Pettrovich. However, when you run up to rescue him, a voice will tell you that you've fallen for a trap, and that the real Dr. Pettrovich is being held on floor 2. When the speech ends, a pitfall will open up beneath the fake doctor. Be sure to move Snake down when the speech ends so that you race away from the pitfall as it opens up. When it's safe, run to the west room next door. Inside, you'll find access card 6 on the desk. Pick it up, and use it to open the southern door and return to the hallway. You'll still be on the same screen. If you're short on supplies, it's a good idea to head east first and visit room A3. Otherwise, open the door to the west to room A1.
A3: Assuming you chose to stock up on supplies, approach the open door in the upper right corner. Inside, you will find ammo and rations on the right, and a set of plastic bombs on the left, and a column of rolling barrels between them. At this point in the game, the barrels should not pose a huge challenge to you. Exit and enter the room as many times as you like to stock up on items. If you've reached four stars, you may notice that it will take a little longer to max out your inventory than it used to. When you are finished, you can return to the west and open the door in A2 with card 6 to reach A1. (If you happen to explore the southern route, you will return to C3 via a door which requires card 3.)
A1: When you arrive, you will be attacked by a man who calls himself Fire Trooper.
Once Fire Trooper is defeated, you will be able to access the elevator behind him. This time, you'll want to ride it up to the second floor. Be sure not to miss it, because if you do, you'll have to cross the roof and drop back down to the basement or first floor to try again.
You should have just recently rescued Dr. Pettrovich's daughter Elen, and traversed the darkened tunnel of pitfalls. You will arrive in C1.
C1: When you arrive from the tunnel, you will have easy access to an open door on the left. Inside are two boxes of ammo. With no guards inside or out, you can easily stock up in no time. Approach the upper right side of the room carefully. As you clear the crates, a pitfall will open up. Let it open all the way, and then hug the right wall as you punch it, looking for a weak point. When you hear the right sound, place a plastic explosive on the wall and blast a hole to room C2. Don't forget to equip your gas mask.
C2: Back in the gassy area, you'll need to take the upper left path to room B2, where you will quickly turn left and continue to B1.
B1: The door that you pass along the way can be opened with card 5. Inside are rations. Collect as much as you need, then continue left, up and back around to the right to B2. From there, travel straight through the hallway until it leads you up to A2.
A2: Return through the door that lead to the fake Dr. Pettrovich, through the open door to the west, then south, and back out to the left side of the room. Open the left door with card 6 to A1.
A1: This is the room where you fought Fire Trooper. Run up to the elevator, and take it back up to the second floor.