Makimura Koutarou spends a normal life as a student. One day in July, near the beginning of summer break, he makes up his mind to confess his love to Serizawa Kaho. However, when he wakes up the next morning, he finds out that he has been transferred to the 1st of September, the end of the summer break.
Still confused, he goes to his school, where he finds his classmates in deep sorrow. Gradually, he realises that Kaho had indeed become his girlfriend... and that she was killed in an accident on the 31st of August.
Returning to 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 try to defy her severe fate... and so a long summer vacation beyond space and time begins.