Swimming instructor, Young-eun is a single mom raising her 7-year-old daughter, So-hyun. However, a series of tragic events happens as So-hyun's puppy falls from the apartment and dies, and So-hyun bullies other kids in school. Despite her mom's earnest efforts to address So-hyun's mental disorder, her peculiar behavior persists. Twenty years later, Min, who works in handling unclaimed 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.