The game features a main set of storyline missions, as well as many optional tasks that can be performed for rewards (typically assassination or object retrieval missions). The game features the 'A-Life' AI system, in which NPCs will roam the world and perform their own unscripted activities. In the game, the player must deal with radiation hazards, areas distorted by the explosion and unexplainable by science known as anomalies, as well as enemy stalkers such as bandits or Army soldiers. In S.T.A.L.K.E.R, players control a 'stalker', a scavenger searching the Zone for valuable artifacts. The year is 2012 6 years after the Chernobyl Nuclear Power Plant suffered a mysterious second meltdown, leaving the 25km radius area known as the Exclusion Zone a barren wasteland filled with mutants, strange inexplicable anomalies and artifacts with strange properties. There are also several underground maps such as the X10-X18 series of experimental laboratories. It features large open maps, populated by mutants, friendly and enemy NPC characters. The game is a non-linear, open world shooter, which falls heavily on the FPS side of the FPS RPG genre. A prequel, S.T.A.L.K.E.R: Clear Sky, was announced by GSC Gameworld in 2007 and released to mixed reviews. S.T.A.L.K.E.R.: Shadow of Chernobyl is a first person shooter developed by Ukrainian developer GSC Gameworld, and published by THQ in 2007.
