The Earth is under threat by the Oni, a group of aliens who promise to leave if a human can win a one-on-one game of tag against Lum, the beautiful daughter of their leader. Unfortunately for high schooler Ataru Moroboshi, he is chosen as the "lucky" player, but he quickly realizes how impossible the task is due to Lum's extraterrestrial powers. However, Ataru is motivated by the promise of marriage from his childhood friend, Shinobu Miyake, and manages to catch Lum off guard, though he mistakenly grabs her bikini top first. Despite achieving his true goal, Lum misunderstands and ends up moving in with Ataru, who desperately tries to shake her off while reconnecting with his desired fiancee. After Ataru's heroic feat leads to disastrous consequences, his luck remains uncertain.