2000 years ago a priest had trapped an oni (Japanese monster/demon) in a small box, and now the seal is about to fade away. Ranma, Soun, and Genma travel to a temple to replace the seal on the box that contains the evil Oni. Just as the new seal is about to be placed on the box, Happosai arrives and blows his nose on the seal allowing the Oni to escape. The Oni makes it's way to the Tendo home and begins to possess various people before finally settling in Kasumi. The only way to drive the Oni out of someone is to punch them, but no one is willing to hit gentle Kasumi. Sensing a great evil in it's presence, the Oni possesses Happosai and powers up to full strength.