Players explore indoor and outdoor facilities in order to complete a usually linear series of tasks (usually of the "FedEx" or "get to Point B" varieties, though other tasks include disarming force fields, enabling a radio transmitter, or fixing various broken machinery), while clearing the area of a chain of progressively more powerful creatures. Occasionally, other NPCs fight alongside the player, but, especially in the case of characters central to the plot, they are invulnerable and can be left to kill while the player recharges his flashlight or hunts for items.