While on vacation in Florida, middle school science teacher John Martin (character actor Herb Edelman, "The Golden Girls") sips from the legendary Fountain of Youth. From that point forward he randomly morphs in and out of a 12-year-old version of himself (Robbie Rist, Cousin Oliver from "The Brady Bunch"), often at inopportune times. He tells his wife Marjorie (Joyce Bulifant, Marie from "The Mary Tyler Moore Show") and son Ricky (Mike Darnell), who pass "Little John" off as his nephew. At school, his students and principal Miss Bertha Bottomly (Olive Dunbar) begin finding Mr. Martin's behavior suspicious. Attempts to cover his secret lead to comical situations. While he looks for a cure, he finds his experience as "Little John" helps him better relate to children. "Big John, Little John" was produced by Sherwood Schwartz and ran for a mere 13 episodes in 1976 before being canceled.
The first episode opens with John Martin walking with his wife Majorie and son Ricky in Florida's Ponce de Leon Park. Overheated, he takes a break to scoop up and handful of water from a spring. He returns to his classroom at Madison Jr. High School to tell the students about his vacation hiking in the Everglades and Ponce de Leon's search for the Fountain of Youth. School principal Miss Bottomly reminds John, known to be forgetful, about his final interview for head of the science department. Alone and without notice, John morphs into a 12-year-old version of himself, to his confusion. He realizes he must have drunk from the actual Fountain of Youth. John races home to struggle to have Majorie and Ricky believe the situation. Still his little size, he goes to school the next morning as "Little John"- John's visiting nephew. At home, John racks his brain figuring out a solution to the problem, fearful he may always be that way. As his important interview approaches, John realizes he may have to go as Little John. Seeing things from a child's perspective may have more benefits than he realizes, however.
