STALKER Trilogy - Clear Sky, The complete Ironman #4. Anomaly. 

The story as told by the three STALKER games, chronologically using Stalker Anomaly 1.5.2
The story begins with Clear Skies. Scar, a Mercenary has survived 2 Blowouts in the Great Swamps. The Clear Skies faction, who want to work with the Zone not against it, believe the blowouts are becoming stronger because a group of STALKER's are heading toward the centre of the zone, led by a man called Strelok, the Zone feels threatened and is trying to defend itself. The group must be found and stopped.
This part of the story takes place in Warfare Mode, first introduced by the Clear Skies game.