The positive identification of the murder weapon leads Thomas down a rabbit hole of World War II memorabilia collectors. While Thomas's pursuit turns up solid leads in the case, it also leads him straight into danger.

The investigation has fallen off track, and Thomas is back in Germany. Helene has bigger things to worry about. A criminal she put behind bars has been released and is out for blood — Helene's blood.

The detectives' leads are drying up. The pair moves their investigation from land to the depths of the sea to investigate a shipwreck from 60 years ago.

Personal and professional boundaries are blurred between the detectives. Meanwhile, the pair pieces together all of the clues to make a shocking discovery about the killer and his motives.