After the trip from Black Station, you will finally arrive on Polis.
This is the final chance to purchase any desired equipment. You do not need to keep any military grade bullets, but if you wish to use them later, you can keep 20 for each of the heavily armored enemies you need to kill, or hold a maximum of 100 in reserve. Excess ammo should be spent on purchasing the heavy suit, a better weapon, or a choice of ball bearings or shotgun shells. You won't find ball bearings later, but there is still plenty of other ammunition.
You will get a chance to select new weapons later after getting through this chapter.
Once you are finished shopping, head towards the ranger and speak with him. After a cutscene sequence, you will be brought outside.
This is a short level, and there are two paths to choose from. Combat will be light, but there are watchers that will attack.
To the left is a dead body with some ammo and filters.
The path to the right leads above some irridated water and a small sewer. It allows you to evade most enemies.
The path to the left is shorter and faster. It brings you closer to the library, but you will have to run over an unstable platform, and dive into a sewer before a demon attacks. When you first climb out, you will see some stairs, but should watch for falling debris. Take cover, and once the debris lands, run to the library.
This level requires escorting the two rangers into the library. After a demon tries breaking down the door, you need to run into the first hall. You will need to proceed to the next floor, but the door is locked. First, collect supplies from the dead body to your right, then head up the stairs. You can collect more supplies by turning right and searching the upper floor. To unblock the door, head into the room to the left of the main doors. Push the door open, and shoot the support blocking the main door to allow the rangers to proceed.
There is a second set of doors to open. When asked to assist, head to the side room to the left, and smash open the door with your knife. Climb through the hole in the wall. To open the main doors, look at the ceiling, and shoot the chandelier to open the doors.
The third room seems empty, but a librarian will run as soon as you spot it. Miller will give a description to handle the librarians. When they spot you, look directly at them, and stand still. As long as you look at them and don't attack, they will eventually retreat. After a demon attacks, you will have to find the archives by yourself. climb the stairs to the right, and enter the room at the back corner.
When you start, there are two bodies to the sides, providing some ammo.
Head down, but try not to alert the librarian. If you do, look them directly in the eyes for several seconds until they walk away. If it crouches and begins to growl, back away slightly. If you provoke them or if they become aggressive, you will have to run if you can't use 20 shotgun or military grade rounds.
On the lower floor, take a right turn. there will be a librarian approaching from the right, and behind him will be a spare filter. To continue, take a left, and collect the filter in that room. Continue to the next room, and enter the bathroom. You should see a hole in the wall with shotgun shells. Ignore them, and turn right and head up the stairs.
On the upper floor, you should encounter a librarians approaching from the left, followed by a second from the right. If you need to take cover from them for a few seconds, run to the end of the corridor and drop into the hole to collect some ammo and a filter. To advance, enter the hole in the wall, and push open the door. The last librarian will appear from the hole from below. Continue along the second floor (being careful not to fall into the holes), and head to the elevator.
This will start a scene, where the elevator starts speeding downward.
Once underground, you can safely remove the gasmask.
The first room has a librarian. To sneak past it, head to the left, and approach from the right, avoiding both the glass and creaky door. You may want to stare it down to cause it to leave. At the end of the room is a spiral staircase, which will collapse easily if you stand still.
The next room requires climbing across a series of pipes until you reach a small vent.
After the vent is the lair of the librarians. You can sneak past them easily as long as you don't get too close. In the first lair, climb the stairs to the right. When you need to cross a gap, jump over the boards (since they will collapse if you try walking on them).
In the second lair, sneak around the right of the librarian, and climb tot he second floor. Collect the bullet on the tray above the hole, and it will fall down on the librarian. Search the office to the right for the final set of supplies, and enter the door in the corner to search the archive room for the D-6 folder.
Driving to Sparta
This will be a short and safe mission. After a ride to sparta base, you are given a choice of the best weapons, which you may take for free. Weapons include a heavy automatic shotgun, a volt driver, and the other most powerful weapons. As there is no other store, you may pick the best weapon available.
At the upper floor, you may speak with Khan for a moral point. When you are ready, speak with Miller about the plans to reach D6.