Based on the classic adult manga of the same title, this film depicts the love between a stepbrother and sister discovered one weekend when their parents go away. The relationship must come to an end, but not before it changes their lives forever.

When Ami and her step-brother, Hiroshi embrace one another in bed, the phone rings. A year later, Ami lives with Hiroshi and his wife, Mariko, but there is an undeniable tension between Ami and Mariko. One day, Mariko gets mad at Ami.

Ami, raised in China, is a high school student who practices Chinese martial arts and swimming. She comes back to Japan because of her mother's remarriage. Hiroshi, a careless college student, likes to take women’s pictures secretly. Hiroshi goes to a girls swimming pool locker room to take photos with his friend, Takimoto. When he gets home, Hiroshi sees Ami whom he saw at the locker room. Ami's mother and Hiroshi's father gets married and they go on a honeymoon, leaving the two alone in the house. Whenever Hiroshi tries to get near Ami, her martial arts prevents him but gradually they become closer…