Junior high school science teacher John Martin, on a vacation in Florida, drinks from the legendary Fountain of Youth allegedly discovered by Ponce de Leon. A few weeks later, he transforms into a 12-year-old version of himself. The change is temporary, but from then on, John proceeds to randomly switch back and forth from "Big John" to "Little John" and back again at unexpected moments. John's wife and son were in on the secret, explaining "Little John" away as their nephew, but the struggle to keep John's secret made up most of the show's humor. Miss Bottomly was the matronly school principal who harbored Dr. Bellowsesque suspicions about John.