A series of murders are occurring in a small town. They have a mystical, ritualistic nature. The investigation reaches a dead end, and locals say a mermaid, the subject of many legends, is to blame. Yuri Bakhtin, a specialist in serial killers, is sent from Moscow to assist the investigation. He doesn't believe in mermaids, but the more he immerses himself in the case, the more he begins to doubt the veracity of his beliefs. In addition to the investigation, Bakhtin must also deal with family issues. It turns out he lived in this town many years ago, and not only has old friends there, but also his ex-wife. As the investigation progresses, the list of suspects only grows, including people close to Bakhtin. Navigating the law, family problems, and obstacles from the antagonist, Bakhtin tracks down the killer.