Swimming instructor Young-eun is a single mother raising her 7-year-old daughter, So-hyun.

Young-eun learns So-hyun bullies other kids in school. And despite Young-eun’s best efforts, So-hyun’s violent tendencies continue to spiral out of control.

Twenty years later, Min, who handles cases of solitary deaths, meets a new colleague, Hae-young, and they form a close bond. However, as Hae-young warmly integrates herself into Min’s life, Min begins to feel uncomfortable and suspicious about her intentions.