A young girl is found dead in Seryong Lake, a reservoir in a remote South Korean village. The police immediately begin their investigation. At the same time, three men - Yongje, the girl's father, and two security guards at the nearby dam, each of whom has something to hide about the night of her death - find themselves in an elaborate game of cat and mouse as they race to uncover what happened to her, without revealing their own closely guarded secrets. When a final showdown at the dam results in a mass tragedy, one of the guards is convicted of murder and sent to prison. For seven years, his son, Sowon, lives in the shadow of his father's shocking and inexplicable crime. When Sowon receives a package that promises to reveal at last what really happened at Seryong Lake, he must confront a present danger he never knew existed.