Kang Hyun-se is a gangster of the Hanbando Group who inherits the Maru Kindergarten after his estranged mother dies from a heart attack.
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.