The show's main character was 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 León.

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 (Olive Dunbar), do become suspicious that something unusual is going on.

The Martin family explain the younger John as their nephew, staying with them.

To make the two actors resemble each other more closely, Rist's blond hair was dyed brown, while Edelman wore a hairpiece that partially covered his baldness, though was absent during the credits sequences.