Go Mi-nyeo (Park Shin-hye) has lived at an orphanage with her twin brother, Go Mi-nam (also played by Park Shin-hye), since childhood, after their father died. Years later, her brother becomes a music idol, and Mi-nyeo lives in junior nun dorm to reach her dream of becoming a nun. Her idol twin brother Mi-nam, successfully wins the "A.N Entertainment" vocal auditions for musical band "A.N.JELL," but Mi-nam must leave in order to correct his botched plastic surgery in the United States. Ma Hoon-yi (Mi-nam's manager, played by Kim In-kwon) approaches Mi-nyeo to pose as Mi-nam for a month till he recovers. Mi-nyeo is against it at first, but eventually agrees, as a path to fame would give the twins an opportunity to find their mother. Mi-nyeo, posing as Mi-nam, joins the band A.N.JELL and meets its members: Hwang Tae-kyung (Jang Keun-suk), Jeremy (Lee Hong-gi) and Kang Shin-woo (Jung Yong-hwa).
At first, Tae-kyung does not like Mi-nam and makes her life hard, while Shin-woo and Jeremy are friendly to her. Tae-kyung discovers that Mi-nam is a girl and threatens to reveal it; however, Mi-nam manages to win him over, and he allows her to stay, while fighting his feelings that are beginning to develop. Shin-woo discovers her true identity before anyone does but does not reveal it and develops feelings for her; Jeremy, thinking Mi-nam is a boy, unknowingly starts to fall for her too. She ends up finding love with Tae-kyung. The story follows Mi-nyeo, who has to keep her secret away from the cameras while performing with A.N.JELL, and her journey in discovering her mother's past, which is somehow connected with Tae-kyung.
Jang Keun-suk as Hwang Tae-kyung
Jung yong hwa as kang shinwoo
Park shin hye as go min yeo and go mi nam
Lee hong ki as jeremy
Kim In-kwon as Ma Hoon-yi
Kim Sung-ryung as Mo Hwa-ran
Famous actress and solo singer, and Hwang Tae-kyung's "hidden" mother.
Jung Chan as Ahn Seong-chang
President of talent agency A.N
Jang Keun-suk and Park Shin-hye were both cast first, with director Hong Sung-chang stating that the two were well-suited to convey the "fresh" idol stars concept he wanted. Lee Hong-gi (a singer with the idol group F.T. Island) had been a child actor, and this was his acting comeback after a five year break (his last acting project being Kkangsoon in 2005). Jung Yong-hwa made his acting debut with this role, which he auditioned for whilst preparing to debut with his band CN Blue in Japan.