Stalker: Shadow of Chernobyl → X-18 Laboratory
First, you need to find the code to open the door to the lower level of the lab. The code is 1243, but you can't use it until you find it on the body of a scientist in the far room. The main enemies here are fire anomalies (watch out for the swaying air). A couple of snorks might also spoil the fun. Dodge the flying boxes.
On the lower floor, you'll need to find a key again. The corridors here are swarming with poltergeists and flying boxes, and in the large hall where another dead scientist lies, there's a lone pseudo-giant running around—it's best to shoot him through the doorway while he's hopping around the room, stretching his legs. Don't forget to go up the stairs and examine the crate with valuables. There's nothing of interest in the refrigerator.
Save and open the door with the code 9524. Kill another poltergeist, look at the mysterious flasks with amazing creatures inside, and go up to the control room, where you'll experience your first flashback—a memory from the past.
Don't forget to pick up the folder from the table. Climb back out the same way. Four soldiers shouldn't pose much of a problem. The army has attacked the factory, so once you reach the surface, it's best to try to escape the other way—to the second floor and down through the roof.
You need to take the documents to the bartender, but the passage to the junkyard is blocked. You'll have to make a detour and return to the Cordon. Show the documents to Sidorovich, then take them north to the bartender.
The next stop is Yantarnoye Lake, where the X-16 laboratory turns unwary stalkers into zombies. You can get there through the Wild Territory.