An alien race known as the Oni arrive on Earth to invade the planet. Instead of taking over the planet by force, the Oni give humans a chance to fight for the rights to the planet by taking part in a competition. The competition is a variant of the game of tag (known as "the game of the Oni" in Japanese), in which the human player must touch the horns on the head of the Oni player within one week. The computer-selected human player is Ataru Moroboshi, a lecherous, unlucky and academically unsuccessful high school student from the fictional in Nerima, Japan, and the Oni player is Princess Lum, daughter of the leader of the alien invaders.