Nao is a married woman who has been married to her husband for several years. Her husband has become colder than when they first got married, and she feels lonely as the distance between them grows. In an attempt to repair their relationship, Nao plans a dinner for her husband's birthday. Wanting to look as pretty as possible on that day, Nao happens to see a flyer for swimming lessons and decides to attend. At the swimming lessons, she gradually becomes Kokoro to the instructor, Yuki, whose kindness is different from that of her husband. However, Nao had no idea that the swimming lessons for her husband would drive the couple crazy...