Chaeyoung's quiet high school life gets turned around when her single father remarries, to a 30-year-old woman named Myoui Mina. Chaeyoung doesn't like her new stepmom at first, but things don't stay that way for long.

Following a conversation that delves into the topic of intimacy, Mina proposes the idea of forming a 'sexual agreement.' Little did she know, that her agreement with Chaeyoung for a casual encounter, would have a profound impact on their lives.