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 computer-selected human player is Ataru Moroboshi, a lecherous, unlucky and stupid high school student from the Japanese city of Tomobiki, and the Oni player is Princess Lum, daughter of the leader of the Invasion force.

It seems that Lum's grandfather made an agreement that should he have a daughter, she would be married to a traveling merchant that he met. Now, the merchant has come to collect due. Ataru is tricked into thinking that Lum wants to leave, and so, in a moment of anger, says he hates her. Arguments erupt, and Lum decides that she must know the truth of Atarus feelings. As for all the questions of importance for the Onis, a game of tag begins, one that Ataru can win simply by telling Lum I Love You. However, in the end his pride may end up dooming him. So begins the series of events that will at long last decide the future and fate of Ataru and Lum.

Lum, Ataru, Mendou and Shinobu visit an inn which Ryuunosuke and her father have opened. But when it turns out to be on a haunted desert island, the stage is set for some serious silliness!