One of the reasons GSC decided to move towards a prequel rather than a simple expansion pack is their desire to clear up some of the story a bit and explain the area around Chernobyl and the factions inhabiting it a bit better. As Strelok, you'll have access to eight factions total, with one new one in the mix. The story really gets going when Strelok finds the passage into the inner areas of the Zone. When the defenders there get wind of this, they begin pushing new anomalies out into the rest of the Zone, totally screwing with the current hierarchy of the Zone. This throws everything into disarray causing fights between all of them as they try to move around and settle themselves in the area.