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.

A middle-aged genius goes to college for the first time.

Posing as a man over 70, a lecturer (Clifton Webb) enters an old-folks home to prove age is a state of mind.