The story starts on September 11th, 2018. Hayato Hino is a high school student at Ryuusei Academy in Tokyo, and is a member of the school's Second Science Club. That day, he goes to the roof of the school to practice his "Dragon Unit", a flight pack. Just as he's about to test it out, he sees a girl falling out of the sky, wearing a similar device. He tries to save her, but fails and falls to his death. However, several seconds later, he finds himself having gone back in time a few seconds ago (though he has no memory of it) with the same incident occurring.

[From Wikipedia]