It'd been decades since the monsters were run off. No one ever found out why they appeared and where they disappeared to. But now they've returned to the city. Monsters are now attacking at night and hiding during the day. You have time to clear the city before dawn before the monsters can hide again only to reappear in a new place.
Control keys:
F - Pick up weapons, ammo
1,2,3 ... Change weapons
WASD - Character movement
SPACEBAR - Jump
SHIFT - Sprint
Esc - Exit to the menu
LKM - Shoot