The set-up: Ataru Moroboshi is a below-average high school student with an above-average interest in girls. Through a random selection, he is chosen as Earth's champion when an alien race called the Oni decide to conquer Earth. Ataru is given 7 days to win a tag race with the Oni commander's daughter, Lum. To win, he has to grab her by the tiny horns on her forehead. Given that Lum is a 17-year-old hot chick in nothing but a tiger-striped bikini, Ataru is eager for the chance, until it turns out that Lum can fly. No one, including Ataru's girlfriend Shinobu Miyaki, really thinks he can pull it off - but since the fate of the Earth is resting on him, Ataru has a huge support system. Six days pass with no luck, and the night before the final day, Shinobu makes the ultimate sacrifice and tells Ataru that if he does win, she will marry him.

Motivated at long last by something that directly affects him - the prospect of being able to cop a feel on his girlfriend - Ataru uses pretty much the dirtiest trick he can think of, and manages to snag Lum by the horns. Unfortunately, his cry of "for my marriage!" is misinterpreted by Lum - especially given that, on her planet, proposals are delivered while grasping horns - and Ataru ends up with a feisty, quick-tempered, and entirely dedicated new "wife". Consequently Shinobu and all his friends want him dead.

From here on out the series develops; initially planned as a series of unfortunate events to the unlucky Ataru and his long-suffering girlfriend, Lum was so popular that the series was rethought after the second chapter was published and Lum was written in as the second main character. The bulk of the stories center either around Lum's strange space gadgets, Ataru's dedication to girl hunting, or the occasional ghost/monster/mythology story. The characters do deepen some, but for the most part it's not a character-driven series, but a basic sitcom.