Makimura Kotaro spends a normal life as a student. One day in July, near the end of summer break, he makes up his mind to confess his love to Serizawa Kaho. However, when he wakes up in the next morning, he finds out that he was suddenly transferred to the 1st of September, the end of summer break.
Still being confused, he goes to his school, where he finds his classmates in deep in sorrow. Gradually, he realizes that she had become his girlfriend… and that she had been killed by an accident on the 31st of August.
Back at the day before he got acquainted with Kaho, he tries to find a way to change this awful future. Time-travelling between the future and past, he continues to defy his severe fate… and the long summer vacation beyond space and time started.