In S.T.A.L.K.E.R, the player assumes the identity of an amnesiac "Stalker", an illegal explorer/artifact scavenger in "The Zone", named "The Marked One". "The Zone" is the location of an alternate reality version of the Zone of alienation surrounding the Chernobyl Power Plant after a second, fictitious meltdown, which further contaminated the surrounding area with radiation and caused strange otherworldly changes in local fauna, flora and the laws of physics.
The game begins with an unconscious, wounded stalker (the player character) being brought to Sidorovich, a black-market trader operating inside the Zone. Sid is able to save his life, but the wounded stalker is amnesic; the only clues to his identity are a tattoo on his arm of the acronym "S.T.A.L.K.E.R." and his PDA which contains only one entry in the to-do list: "Kill Strelok."