Although not related by blood, Seo Gun (Gong Yoo) and Seo Ha-neul (Sung Yu-ri) are legally siblings after the marriage of Gun's father and Ha-neul's mother. But after the deaths of their parents, the siblings are sent to an orphanage where they tearfully part once they get adopted into different families.
In Korea, Ha-neul is a given a new name, Park Hae-won, and grows up in a wealthy, loving family, so much so that she finds it suffocating. Her adoptive mother (Sunwoo Eun-sook) treats Ha-neul like her dead biological daughter, while her adoptive brother Park Tae-won (Yoo Ha-joon) is obsessed with her.
In Australia, Gun lives a life of destitution with his adoptive father Goo Kyung-taek (Lee Ki-yeol) and adoptive sister Goo Hyo-joo (Lee Yeon-hee). He becomes a small-time gangster whose main talents are swindling and fighting. When he hears from his best friend Goo Sung-chan (Kang Sung-jin) that his sister Ha-neul was adopted by a rich family and is now living like a princess, he decides to return to his native country in order to con Ha-neul into giving him money. Hyo-joo, who's nursed a crush on him for a long time, follows Gun to Korea.
Meanwhile, Ha-neul lives every day wanting to get away from her adoptive parents and brother. She secretly works at an aquarium and constantly gets into quarrels with the curator, Kang Dong-ha (Namkoong Min), who starts to like her. Then one day, Gun suddenly appears in front of her. She'd never forgotten him and like she'd imagined a million times in her dreams, her "real" brother has come back to get her, just as he'd promised. Now worlds apart from each other, they reunite after 15 years of separation. Gun initially just wants to swindle money from Ha-neul, but he can't help protecting and caring for his little sister. As the two grow closer, their relationship and feelings become more complicated, and they come to realize that their mutual affection is something more than that of a brother and a sister.