Ataru Moroboshi is a hopeless, lecherous high schooler chosen at random to play a game of tag against invading alien Lum, with Earth's fate riding on the outcome. He wins by grabbing her horns and shouting a marriage proposal meant for his girlfriend Shinobu Miyake, and Lum takes him at his word, moving into his house as his self-declared fiancée. What follows is a running collision between Ataru's wandering eye, Shinobu's fury, and a steady stream of alien, demon, and yokai visitors drawn to Tomobiki by the chaos.