The story is about a lecherous boy named Ataru Moroboshi who chases after every pretty girl he meets, he is choosen to defend the earth against Lum's people the Oni (Japanese Ogres) in a game of tag. Ataru wins and screams about how he will marry his girl friend, unfortuneately Lum mistakes this as a proposal to her and she accepts. Eventually Lum moves into Ataru's closet and is ever present in his life much to Ataru's disapointment. Truthfuly Ataru really does love Lum more than any other girl, if he could just keep his hands off other girls. Lum is always zapping Ataru with electricty every time he tries to ask another girl out.
Other equally weird things happens in Ataru's life, Lum's alien friends start showing up many of them are female at which Ataru goes after them as well.