Tacey and Harry King are a suburban couple with three sons and a serious need of a babysitter. Tacey puts an ad in the paper for a live-in babysitter, and the ad is answered by Lynn Belvedere. But when she arrives, she turns out to be a man. And not just any man, but a most eccentric, outrageously forthright genius with seemingly a million careers and experiences behind him. Mr. Belvedere works miracles with the children and the house but the Kings have no idea just what he's doing with his evenings off. And when Harry has to go out of town on a business trip, a nosy parker starts a few ugly rumors. But everything comes out all right in the end thanks to Mr. Belvedere.