Kim Jongin is officially the boss of fifteen-year-old Moon Somin. After a mysterious fire burned down Somin's home, she is left with nothing but a half-burnt picture frame, the clothes on her back, and her neurotic mother. With Somin's current circumstances, her mother arranges for them to be live-in maids to a wealthy family in Gangnam. The family that jump started her mother's cleaning career. The family of Kim Jongin.

Somin's arrival to the Kim household initiates the start of an eerie, tempting, seductive rollercoaster ride of discovery. And it's all thanks to the boy she now refers to as "Master".