Go Eun-byul and Lee Eun-bi (both played by Kim So-hyun) are identical twins, separated after one is adopted at the age of 5. Eun-bi lives at the Love House, an orphanage in Tongyeong, South Gyeongsang Province, where the younger residents look up to her as a mother figure. However, she hides the fact that she is bullied at school by a gang of mean girls led by Kang So-young, while teachers turn a blind eye.
On the other hand, Go Eun-byul is studying at Sekang High School, the most prestigious private high school in Seoul's Gangnam District. Eun-byul's best friend is Han Yi-an (Nam Joo-hyuk), the school's star swimmer and is in love with her since childhood.
However, only Eun-byul is aware of the other's existence. Unlike the cheerful Eun-bi, Eun-byul is prickly and secretive.