Hyun-Bin has been surviving on the streets by hiding the fact that she's a girl. In a tragic turn of events, her beloved little brother dies and Hyun-Bin runs away from the begging, from the gangs - straight into the clutches of a rich family. Her life might have become better but it's far from rosy: she has to undergo strict training every day and guard the family's youngest member, innocent and princess-like Yae-Ha. There are only two rules, Hyun-Bin has to protect Yae-Ha with her life and no one must ever find out that she's a girl. It's a lonely and cold way of life but a fateful encounter with the delinquent Yun-Ha becomes a catalyst of changes to come, where loyalty and a sense of duty clash with feelings of love...