"I'm going to take revenge on them."

Yuusuke Midorikawa, a high school boy, has a girlfriend named Kayo Yoshioka, who was his childhood friend. After many years, he confessed his long-standing feelings for her and they officially became lovers; but it seems she's becoming estranged from him lately. One day, while he worried over these age-appropriate problems, he witnesses Kayo and another boy who seems to have hit it off with her, leaving a karaoke place by themselves. Yuusuke, unable to put his feelings in order, is met the next day by a pretty classmate named Kotone Shiraki, who introduces herself as the girlfriend of Iketani, the boy he saw with Kayo.