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. However, only Eun-byul is aware of the other's existence. Unlike the cheerful Eun-bi, Eun-byul is prickly and secretive. One day, on a school field trip to Tongyeong, Eun-byul mysteriously disappears. At the same time, Eun-bi is unjustly expelled from her school and jumps off a bridge in a suicide attempt. Ten days later, the injured and amnesiac Eun-bi is mistaken for her twin, Eun-byul, and her adoptive mother takes her home to Seoul, where she starts living as Eun-byul. However, her memory soon returns. Eun-bi faces many challenges as Kang So-young transfers to Sekang High School and tries to reveal her real identity, as well as using her Dad's authority to make Eun-bi transfer from the school. Luckily, Gong Tae-kwang (Yook Sung-jae), Eun-bi's classmate and the school troublemaker, protects her whenever So-young tries to bully her. Eun-bi also stands up to So-young and disproves her claims about her identity, as well as ruining her plans to have her transferred. Eun-bi tells Tae-kwang her real identity, and Tae-kwang becomes the only person Eun-bi fully trusts. He eventually begins to develop feelings for her.