Masato Wakamatsu, 16 years old, is working at the beach one summer. Having made a bad impression on his classmate/crush Miyuki Kashima, he makes a pass at another pretty girl. To Masato's shock, the girl turns out to be his younger stepsister Miyuki (15), who has been living abroad with their father for the past six years.

As school progresses, Masato starts dating Miyuki Kashima. He and his sister Miyuki are living alone together, and he is troubled by his conflicting emotions: filial and romantic feelings toward a sister who may or may not know she is not related to him by blood.^[4