When the ONI, a mysterious race of Aliens, come to repossess the Earth, they give us one chance to save our planet; a champion, selected at random, must defeat their princess, Lum, in their national sport--TAG! Unfortunately for Earth, the champion turns out to be Ataru Moroboshi, a lecherous lad whose lack of luck is legendary.
When Ataru lays eyes on the curvaceous Lum, he readily agrees to take on the task of touching the tips of her tiny horns. Alas, when the contest begins, he finds out that the Oni forgot to mention one small matter--Lum can fly! When all seems lost, his girlfriend Shinobu promises to marry him if he wins. Inspired to new lows of deviousness by the prospect of getting just to first base, Ataru pulls it off. Unfortunately, Lum misinterprets his victory cry as a proposal of marriage to HER, and promptly agrees. So Ataru ends up with Lum living in his closet, planning their wedding, and zapping him with high-voltage shocks if he even looks at another woman; Shinobu isn't sure if she's been jilted or she's been saved from a life of misery; his Lum-crazed schoolmates are plotting to get him "out of the way"; and his parents are loudly wishing he's never been born.