Jan Lessing, a chief inspector in Cologne, has problems: he feels unwell, a new colleague is making his life difficult, and he needs to find a murderer who leaves a red heart at the scenes of his crimes. The perpetrator seems to know Lessing: he sends him emails and hangs red plastic hearts all over the city as if they were a secret sign. Then a third murder occurs, but there is a witness. While Lessing hopes to find a clue, the killer is planning his final act: he wants to kill Jan Lessing.