Jenny and her friend Old Ben are at the park as the other kids play baseball. Jenny is unable to play because of a leg brace. Old Ben gets up to bat and hits the ball several miles out of sight. The kids, used to this, complain that he's using his magic. Jenny suggests they play "Spaceman" and Ben ends up playing the Martian as the other kids say he's the only one of them who can transform. He goes behind a rock while the kids pretend to be astronauts on a mission, then emerges as an alien monster. The kids pretend-shoot him and he collapses, then changes back to Ben.
Ben and Jenny go back to their shared apartment building, unaware that two men are watching for Ben but seem unsure if he's who they're looking for. As Ben carries Jenny upstairs because of her leg, she wonders why he can't heal it but he says he can't without explaining. When they get upstairs Jenny's Aunt Gann, her guardian, complains that Jenny is late and orders her inside. When she threatens to hit Jenny, Ben stops her and Gann tells him to mind his own business. Ben goes upstairs to his room while Mrs. Gann sends Jenny to bed without supper. The two men visit Mrs. Gann and start asking her questions about Ben: how long he's been living there, if he's ever done anything magical. Jenny sneaks out past them and goes upstairs to warn Ben. He recognizes them from her description and says that they've finally caught up to him. Jenny asks why he's never said anything about himself. Mrs. Gann comes upstairs with the men and knocks on the door. They come in but Ben is gone. Jenny says that Ben wasn't there and Mrs. Gann takes her back to her room while the men leave to search for Ben.
Once alone, Jenny removes a white mouse from her shirt and puts it on the bed, and it transforms back into Ben. Jenny wonders why the men are after him and Ben says that he's sort of a criminal. The men are still watching the apartment and he worries they'll find him no matter what he does. Finally he tells Jenny that he's an alien from another planet, whose true form she's never seen. He explains that the two men are trying to capture him and he has no choice but to find another world to hide on. Jenny isn't happy but Ben has her lay down so he can finally fix her leg: doing so before would have given away his presence but it makes no difference now.
Mrs. Gann hears them talking and calls the men while Ben uses a handheld device to fix Jenny's leg. Mrs. Gann comes in but Ben transforms into a fly so she doesn't notice him. Once she tucks in Jenny, Ben flies out the window and Jenny runs downstairs after him. The two men are waiting and use a similar device to paralyze Jenny.
Mrs. Gann summons the doctor who says that Jenny's leg is fine but that he can't do anything for her and the girl is dying. An upset Mrs. Gann takes the doctor into the next room while Ben returns as a fly, returns to his human form, and cures her. However, the two pursuers are waiting for him, figuring that he'd come back to save her. Ben explains that the men are his subjects, and he is their king. He got bored after the first thousand years of his duties and snuck away. The pursuers want him back on their Council's orders. The Council would refuse to let Jenny accompany Ben, although she wants to go.
Ben appears to have no choice but Jenny whispers an idea to him. He tells his subjects to give him one minute and swears not to try and flee. They agree and go outside, where the doctor and Mrs. Gann are frozen. After one minute they return… and find that Ben has transformed into an exact replica of Jenny. The pursuers have no choice but to take both of them along. As they leave, Mrs. Gann is unaware that Ben's true form is a handsome young man, and her niece will be the queen of an alien planet.