Kang Hyun-se is a gangster of the Hanbando Group who inherits the Maru Kindergarten after his estranged mother dies from a heart attack. He intends to sell the kindergarten and comes to terrorize the staff, only to learn that his mother had stipulated that he has to work as kindergarten teacher for 100 days in order to gain legal ownership.
Meanwhile, a development firm called NK plans to buy the kindergarten's property and begin construction on a project named Rainbow Park, an amusement park for children. Rainbow Park is a dream project of one manager named Jung Il-do, who mentors a young graduate from America, manager Choi Shi-wan, who incidentally shares a past with Yoo Mi-rae, a dedicated and naive kindergarten teacher.
The Hanbando Group, headed by gangster boss So Dong-pa, convinces Hyun-se to fulfill his mother's conditions and then turn over the kindergarten to the gang's control once the 100-day period is completed. This would force NK into giving So Dong-pa a seat in its board of directors.
Hyun-se reluctantly endures teaching at the kindergarten, inevitably falling for Mi-rae and the lovable children. He also discovers the identity of his biological father.
