Kang Hyun Se (Kim Jae Won) grew up without knowing his father. Due to the lack of fatherly love, he was recruited by a gang and became a third rate mobster. After his mother's death, he inherits a kindergarten, but he must go through a hundred days of training in order to get it, so he becomes an assistant teacher under a kindergarten teacher Yoo Mi Rae (Han Ji Min) and they develop romantic feelings for one another. However, the kindergarten property sits in the middle of an proposed amusement park development, called "Rainbow Park". Hyun Se's boss needs the money from the construction company so he is manipulating Hyun Se's hatred towards his father and threatens Hyun Se's father with the kindergarten Hyunseh has. Little does Hyun Se know that Choi Shi Wan (one of the managers who work for Hyunseh's father), who is the ex-boyfriend of Yoo Mi Rae, struck a dirty deal with Hyun Se's boss and is trying to outscore everyone.