Keita Aono goes to school one day, and his class happens to have a new student. That student is a girl named Hinoki Sai, Keita's childhood friend. Later, Keita gets himself lost in an amusement park where many people have mysteriously died. Keita catches up with Hinoki, and the two are attacked by a large monster. Who will save them? The being known as Betterman.
