A hundred and twenty years ago Lum's great-grandfather cheated an alien named Uba out of a promised bride, and Uba has come to collect: too old to marry Lum himself, he means her for his great-grandson Rupa. When Lum refuses, the two carry her off to their homeworld, the World of Darkness. It falls to a game of tag Ataru can win simply by telling Lum he loves her, if his pride lets him say the words before Earth is buried under building-sized mushrooms. This film adapts the final arc of the original manga.