After spending five years at Europol in The Hague, Johan Falk returns to home to Gothenburg, Sweden, to fight organized crime in the special GSI unit. Already on his first day, he has the feeling that something is wrong and his colleagues seem to be hiding something from him. Sure enough, when an armored car is attacked and one of the guards killed, the Swedish police is under pressure for results. In a shootout, the GSI cop Tommy is killed, and the alleged killer is a certain Frank Wagner, who suddenly turns up that night at Johan’s apartment and informs him that he's been working as an informant for the GSI for three years now and that Tommy was his contact. Wagner’s familiarity with the underworld allows the GSI access to special insider information and helps the most effective unit in the Swedish police to score ever new successful mass arrests.