Nao is a married woman who has been married to her husband for several years. - She has grown colder than when she first got married, and feels lonely due to the distance between her and her husband. - Nao plans a dinner for his birthday in an attempt to mend their relationship with her husband. - Wanting to greet the day in as beautiful a shape as possible, Nao happens to see a flyer for a swimming class and decides to attend. - At the swimming class, Nao gradually becomes attracted to the instructor Ariki's kindness, which is different from her husband. - And she had no way of knowing that the swimming lesson for her husband would drive the couple crazy...