The show centers around a forty-year-old middle school science teacher named John Martin (played by Edelman). While vacationing in Florida, he drinks from a spring which turns out to be the legendary Fountain of Youth sought by Juan Ponce de Leon. The water has the effect of changing him into a twelve-year-old boy (played by Rist), and back again.
Martin has no control over when the changes will occur, because he only took a sip of the water, and not a full drink, which explains why the changes are recurring and not permanent. Only his wife, Marjorie (Joyce Bulifant) and son, Ricky (Mike Darnell) know his secret, though Martin's students (who befriend him as "Little John") and his boss, principal Bertha Bottomly, become suspicious that something unusual is going on. The Martin family explain the younger John as their nephew, staying with them. Throughout the series, "Big John" unsuccessfully tries to find a cure for his predicament, but his experiences as "Little John" often give him insight into what his students are facing.