Harry investigates the death of a woman who drowned in a backyard pool. She was apparently struck on the side of the head and her husband claims that her expensive necklace is missing. Leo and Nikki investigate the death of Lucie Philips, a kidnap victim who was taken a week previously. The girl's father tried to pay the ransom but the money was never collected leading the police to believe that the kidnappers may have had other motives. They also investigate the murder of a pizza deliveryman that has similarities with the Philips crime scene. A third murder leads the police to suspect that they may be seeing the work of a serial killer. Nikki's grandmother dies bringing her into contact with her estranged father.

Leo and Nikki find yet another victim who was apparently bound and buried alive. However, they continue to have difficulty finding a common link among all of the victims. The investigation focuses on the editor of a philosophical journal who had an intellectual disagreement with one of the victims but the case is more complex than first imagined. Harry discovers that his drowning victim was pregnant which may have indirectly contributed to her death. Nikki's father is released from prison and promptly disappears.

The team investigate the systematic shooting of a family picnicking beside a river. The case stirs memories of the death of a young woman beside that same river 10 years earlier.

As DCI Butcher moves closer to making an arrest, the team continue to pursue their own lines of inquiry. Amy's past begins to catch up with her and converge with the team's investigation.