Sinb is known by many people as a good girl. She mostly she wants to keep to herself as she strives to be perfect. She loves her girlfriend Umji, who she's been dating for 2 years.

Umji is a really great girl; she's kind, warm, smart and cute. But Umji has a secret that Sinb doesn't know about yet; she has a stubborn sister named Yerin.

Yerin is the opposite of Umji; she's so selfish that she only looks out for herself, and she does what she wants to do without regarding others' feelings... and Yerin is a selfish girl who's gonna try to steal her younger sister's girlfriend.