Six-year-old Ataru Moroboshi once stepped on a girl's shadow during a game of tag, not realizing that among her people, that act counts as a marriage proposal. Eleven years later, having long forgotten the incident and now dating the alien Lum, Ataru is stunned when that girl, Princess Elle, returns to Earth to collect on the promise. As Elle's wedding preparations spiral into chaos, Lum scrambles to stop the ceremony and keep Ataru for herself.