The anime is an action packed adventure, where Rin, Satan's son, fights against demons as an exorcist. The movie is almost completely different. While it starts with some action, it quickly turns into a more emotional movie. True Cross Academy is in the middle of holding a festival, meanwhile Rin, Yukio and Shiemi are busy exorcising a demon train. During the exorcise, something goes wrong and a small demon named Usamaro is released. The exorcists decide to deal with Usamaro after the end of the festival, and he is placed in Rin's care for the duration of the festival. Rin comes to love the small, trouble making, innocent demon as a brother. What he doesn't know is that the festival is actually the resealing of the prison meant to hold Usamaro, the demon from a ancient story who once destroyed a village by making its inhabitants forget about work and only play games all day, except Usamaro believes he was only having fun, and nothing bad really happened. He begins to 'help' Rin and his friends by making everyone forget about their work. As a result, demons begin to invade the city, destroying everything. Rin is faced with a tragic decision, whether to kill Usamaro, his close friend, or protect him from those who would kill him.
