The Oni, a mysterious race of aliens resembling mythological Japanese demons, come to Earth intending simply to invade and take over. Represented by their leader Invader, he tells the people of Earth that they have but one chanceĀ—If Ataru Moroboshi, a teenage boy chosen randomly by a computer as Earth's champion, can defeat Invader's daughter Lum in a game of tag (by touching the horns on her head), the Oni will just pack up and leave quietly.