While on a trip to Florida, John Martin stumbles upon the legendary fountain of youth. Willing to test the waters, John sips a handful of the liquid. The consequences are unforeseeable, he begins to revert to his childhood on certain times, causing a lot of situations for John and his family.
John must face a dilemma when he is chosen to become Captain Hook in the annual school play John where his alter ego, Little John, is to play the lead role in Peter Pan.
John tries a new formula in the laboratory that apparently might cure him from his conversions. A misunderstanding with a baby left in his care has Big John being confused with the infant.
John Martin is detained when a sheriff demands his ID and he cannot produce it. He gets his freedom when his personality changes in prison.
John has summoned Mr. Boswell, Stanley’s dad, to account for his son’s continuous absence to class for five days. Boswell explains that the boy is afraid of crossing in front of the Crabtree mansion, an abandoned house that neighbors believe it to be haunted. John decides to find out who’s behind the Crabtree ghost.
Marjorie and Big john are invited to Miss Bottomly's costume party. But the costume doesn't fit very well when he becomes Little John and changing to King Arthur doesn't help. Meanwhile at school, Coach Adams decides he needs Little John for the basketball team.
On Thanksgiving John has to be a chaperon for a Thanksgiving dance at the school, while they are also decorating. He also has to deal with a girl who his son likes but who is asking little John out.
Big John's voice in Little John's body puts not only Martin in a squeeze but his family as well.
Stanley is again late for the class. The teacher is speaking to his students about the importance of facing up bullies. This leads to trouble for Little John when he's threatened by the class bully, Gasey.
John is assigned to find who's behind the vandalism inflicted on the school walls. Martin is invited to join the school basketball team since he was once a superhero for his old team.
Little John receives a severe reprimand for having played unwillingly a joke on the school principal.
Because of the arrangements for the annual dance that it is held at the school, Ms. Bottomly proposes to professor Martin that he be one of the chaperons. At the same time that the Little John is chosen by his female classmate as companion for that day.