Move out of the initial room and head east - soon you'll notice a passage leading further south; beware, it has a Gauss sniper at the end of it - he can be hard to see, even if you have nightvision - if he doesn't spot you, you can always get out your binoculars and pinpoint his exact position with that. Deal with him and move south.
Move into the next hallway proceed north and then east - soon you'll come to an opening to the north - careful, beyond there is a room which has numerous solders in it. Deal with most of them, but before you proceed that way, move further east; there may be two solderis ahead so proceed with caution. Moving south you'll see another room - the area ahead is very radioactive so watch for the icon and use antirads and medkits (or bandages) as appropriate. Just inside the door, to the south, you'll notice a ladder going down - it'll take you to the bottom floor. Head down and find the stash box and a wooden crate. Search the stash to find some antirads, ammunition, bandages, medkits and the Exoskeleton armor. Break the box to find some good artifacts. Move up again; there is another box here containing some ammunition. Once you got what you need, head back into the hallway.
Move back to the path going north and slowly head into the room - you may have dealt with the majority there, but there is likely still a couple of soldiers remaining. One of the soldiers here is carrying the FT 200M - a solid assault rifle with an excellent firing rate and an integrated grenade launcher. If you still carry the GP 37 or a similar rifle you may consider replacing it; as already mentioned I usually stick to the Vintar. Also, whether you like the FT is pretty much up to personal preference. Move west, into the hallway and follow that south until you come to a room with stairs going up. There are soldiers about so proceed with caution.
Up the stairs you'll find a doorway to the west; there are many soldiers beyond and the doorway can be an excellent choke point if you can manage to lure the soldiers out. If this is the case, you can deal with most - if not all - of the soldiers on this floor. If not, then simply slowly make your way forward and go either left or right and clear out the place. Once done, you are now ready to move on - either to the Wish Granter or to the "secret door".
Both places leads to different endings of the game, so you may wish to explore both. Just make sure that you create a new full savegame before you do, mainly because that if you do have the encoder from Pripyat there is another way to end the game providing the best endings. The entrance to the area with the Wish Granter is to the south and should me marked on your minimap. Refer to the video on how get to the Wish Granter. Depending on what you've done during the game you may get different endings from the Wish Granter; refer to this page for the different endings and how to obtain them.
The "secret door" is located on the nothern side of the floor - you'll find the entrance and a ladder near the rotating light. Once you've climbed the ladder you'll be prompted to open the door using the decoder. Just inside the room near the ladder you'll find another Exoskeleton armor - so you might as well replace your old one, even if it's only slighty damaged - you're not likely to come back here once you move beyond the door. Near the door you can find a metal box containing some goodies. Approach the door and "use" it - a timer will start indicating that the decoding will take 30 seconds; not a huge problem except that soldiers will be teleported into the room you're in - it should be fairly easy for you to use the cover near the door and keep the soldiers at bay until the door finally opens - when it does, proceed through it onto the level transition.
Once you enter this area you should notice about 28-30 PDA's in the vicinity - all Monolith goons who are going to try to stop you from discovering the secret about the Monolith. Your goal is the control room marked on the map. Even though the room appears to be close by the entrance, you'll have to move all the way around the level to reach it since all the other hallways are blocked by grates or fences.
Proceed forward and you'll soon and opening to the south, going into a long hallway - expect alot of soldiers around so be sure to stay in cover and lean out to have a look and possibly take shots at any enemies in sight. To the west there's likely to be a Gauss sniper behind a grate/fence so be sure to deal with him before you start making your way into the hallway. Move east and then south. You'll come across a few room on the way and some of them may have a soldier in it - so be sure to check them out before proceeding; also be sure to break any crates you come upon - some of them have various goodies in them. When you reach the end of the hallway going south lean out and check ahead of you - there may be a collection of soldiers further down to the west, including another Gauss sniper.(A exoskeleton may be by the gauss sniper in a room
Eventually you should come upon the room before the control room. It will also contain a number of soldiers, but since it's rather confined, throwing in a couple of grenades should take care of some of them. When you've dealt with them all, move into the room but don't head into the control room, it's got three Pyrogeists inside it (the nasty type that does the flame thing); you can complete your final goal from the outside. If you do head into the control room, the door will close behind you and that will just make it harder on yourself. However, the Pyrogeists don't seem to appear before the first blue orb thing is destroyed, so it is possible to just damage them uninterruptedly and then have it easier to destroy them while the Pyrogeists are around.
If you look into the control room you'll notice a number of blue orbs - and above them a hologram resembling the monolith. You need to shut down the monolith so start shooting the blue orbs - they will take a few shots to destroy. Once you've destroyed them all the level will end.
You'll see another hologram, this time representing a professor-like person. Now it's finally time to find out what the Zone is all about, what's with the S.T.A.L.K.E.R. tattoo, and why you were been assigned to kill yourself! Well, sort of. Although the hologram will be able to answer some of your questions with decent answers, I don't really blame you if you still feel slightly puzzled afterwards - it does take a while to tie all the ends together to make sense of the story. Read the last section for my personal explanation of the plot.
Your final choice is whether or not you wish to join the C-Consciousness project. If you decide to join them, you'll be treated to a cutscene and the game will end. If you refuse, however, you still got some fighting to do before the show is over. Go to the next page in this case.