The events of S.T.A.L.K.E.R.: Call of Pripyat unfold shortly after the end of S.T.A.L.K.E.R.: Shadow of Chernobyl following the ending in which Strelok destroys the C-Consciousness. Having discovered the open path to the Zone's center, the government decides to stage a large-scale operation to take control of the Chernobyl nuclear plant.
According to the operation's plan, the first military group is to conduct an air scouting of the territory to map the anomalous fields. Thereafter, making use of the maps, the main military forces are to be dispatched.

Despite thorough preparations, the operation fails. Most of the advanced helicopters crash. In order to collect information on the reasons for the operation's failure, Ukraine's Security Service send their agent (protagonist - Alexander Degtyarev) into the Zone. From now on everything depends on the player

The player must traverse through the Zone and investigate the crash sites. As the player progresses he starts learning how the helicopters were shot down. The climax of the story begins when the player reaches Pripyat to find survivors of the helicopter crashes. The player eventually meets Strelok from the original S.T.A.L.K.E.R. game (Shadow of Chernobyl) and learns of the secrets behind the Zone.

The game concludes with the survivors, Strelok, and the player leaving the Zone. Before boarding the helicopter the player is prompted whether he wishes to leave The Zone forever or continue exploring the Zone. If the player decides not to leave the Zone, then the game enters a free-play mode. During free-play mode the player can leave the Zone by NPCs at any time he wants.

The game's ending movie differs depending on the actions of the player during the game. How the player handles in-game missions, and whether certain NPCs are alive or not by the end of the game directly effects the ending sequence.

Link for more information: http://cop.stalker-game.com/