When you start you will be on a ladder automatically climbing to the top. Once you reach the top you are able to move. During the prologue, Hunter will give basic instructions on what to do, telling you to open the door, search crated for items and ammo, and so on.
Go to the metro station lobby. Hunter cannot die during this battle so don't worry about protecting him. If you keep close to him some of the monsters will attack him instead of you which should help.
After you have gotten rid of the monsters follow Hunter outside and there you will be greeted by other allies. A large swarm of mutants will arrive, first appearing to circle your group. One of the mutants will approach, growl for a few seconds then attack. Shoot the enemies as they attack you, switching to the pistol or knife if you run out of ammo. It is possible to die if you aren't careful; but once the gasmask shatters, the cutscene showing the loss of the allies will start.
The game continues, describing the events 8 days earlier. You will be escorted to the gate, where you will meet hunter. After a brief introduction, the mutants will start attacking. Grab the pistol and ammo from the wall locker and wait for the mutants to arrive. The first will drop in the center, and be killed by a scripted combat. This is followed by more mutants dropping in the center, and others trying to break down the wall vents. Save the ammo for the ones in the wall vents, and use the knife on others.