S.T.A.L.K.E.R.: Clear Sky, is the stand-alone prequel for S.T.A.L.K.E.R.: Shadow of Chernobyl, a first-person shooter computer game by Ukrainian developer GSC. The game is set in 2011, one year prior to the events of the original S.T.A.L.K.E.R. game. A group of stalkers, for the first time, reaches the very heart of the Zone—the Chernobyl Nuclear Power Plant—and triggers a cataclysm on the brink of a catastrophe. An immense blast of anomalous energy transforms the Zone: the once reliable and relatively safe roads are no longer so, the landscape is wiped clean by outbursts of anomalies, and previously unknown areas appear on the Zone map. Stalkers and expeditions perish or end up isolated within the lost territories.