Stalker: Shadow of Chernobyl → Dark Valley
The stalker will offer to help him with an ambush: two bandits are leading a captured comrade, a stalker named Semyon, along the road to the factory. Once there, sit behind the bus stop, wait for the bandits to pass, and open fire when the stalker starts shooting. After rescuing Senya, you'll receive a PSO-1 scope and a new mission: free the other captured stalkers.
This level also features a partially stealthy approach—you'll enter the factory where the bandits are holed up, also through a sewer pipe. Avoid the guards, find the spot where the ZIL has fallen halfway into a repair pit, and, with perfect timing, jump into the basement. Carefully and quietly, make your way through the basement to the neighboring building.
Borov is holed up at the far end of the building on the lower floors. Take his key (you'll likely be discovered by then, so bring out your most powerful weapons). If you want, free the stalker in the basement and in the small house behind the gate. Now you have both keys. You can exit the factory via a long passage that starts in the brick building (south of the building) and leads into the valley.
Now you need to storm another factory building—the one where you were lying in ambush with Bullet. Be prepared to shoot a lot or break through. From the courtyard, enter the room on the left, turn right, and go down to the basement. There you'll find a locked door that leads to Laboratory X-18.
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