When Earth faces a threat from alien beings called the Oni, they propose a peculiar condition for leaving - a randomly selected human must win a one-on-one game of tag against Lum, the striking daughter of the Oni leader. The "fortunate" individual chosen is the amorous and unlucky high schooler, Ataru Moroboshi. With a mere 10 days to attempt to seize Lum's horns, Ataru quickly realizes the seemingly insurmountable challenge, given Lum's otherworldly powers.
Driven by a promise of marriage from his childhood friend Shinobu Miyake, Ataru seizes an opportunity to catch Lum off guard. In the process, he mistakenly grabs her bikini top first but eventually achieves his intended goal. Despite the conclusion of the game, Lum misinterprets Ataru's intentions, believing she is the chosen bride. Consequently, she decides to move in with him. The struggling student endeavors to shake off Lum's persistent affections while trying to repair his relationship with his desired fiancée. After Ataru's supposedly heroic feat leads to a calamitous outcome, the question arises whether luck will ever be on his side.