Danny O'Neil [Gabriel Olds] is a 14-year-old boy who is madly in love with his teacher, Miss Peggy Noble [Daphne Ashbrook], while Peggy herself is engaged to the cold-tempered gym teacher Roy Kelton [Rick Rossovich], nicknamed Jackjaw for his constant threat of breaking jaws. Danny endures his school days with little inspiration, watching Lloyd Duffy [Adam Carl], his geeky orphaned friend who lives next door with his uncaring uncle Herb [Harry Morgan] and ditsy Aunt May [Irene Tedrow], work on an experimental growth accelerator to improve their fruit harvest. Danny becomes obsessed with the idea of turning himself into a grown man using the same machine, hoping that a mature look will win Peggy’s affection and end her engagement to Kelton. With his parents away for a week, Danny acts on a reckless impulse and secretly powers up the device, transforming himself into a 30-year-old man.
The next day, Lloyd races to build a machine that can reverse the transformation, but his prototype experiments—tested on tomatoes—fail, leaving him frustrated and increasingly jealous of Danny’s new freedom. Meanwhile, Danny visits the high school and is mistaken for the school’s new principal, Harold Forndexter [Alan Thicke], who is supposed to arrive that morning but is delayed due to miscommunication. Playing along with the mix-up, Danny—now posing as the principal—unveils a lively, student-friendly approach to school life, introducing rules that emphasize having fun and seizing the moment. He captivates the staff and students, including Peggy and the assistant Louisa Horton [Loretta Swit], who are charmed by his fresh, irreverent outlook on education. Danny’s easy charm and unorthodox methods create a buzz around the campus, and Peggy begins to see him in a new light.
Lloyd continues to struggle with reversing Danny’s age while watching his own life feel more constrained; he grows conflicted as he recognizes how different life could be if he could shed his own burdens. Danny’s youthful spontaneity loosens Kelton’s control over Peggy, and she agrees to go on a date with him, despite Kelton’s persistent surveillance. At the school dance, Danny finally convinces Peggy that Kelton isn’t right for her, and she breaks off the engagement. By this point, Lloyd has succeeded in creating a working reversal device, but when he informs Danny, the “Harold” persona fully takes over, and Danny refuses to go back, confessing that he enjoys his new life and the freedom it affords him. He is soon pursued by the police, but manages to escape just as the reversal device is ready, and the two devices and the chase culminate in a tense moment where Danny is forced to revert to his true 14-year-old self.
Peggy, who followed him and witnessed his transformation, realizes that the person she loves is the same Danny she has always known. She chooses to accept his true self by asking Lloyd to turn her into a 14-year-old herself, so she can be with him. In a surprising twist, Lloyd—seeking his own escape from an unloving family life—gives in to temptation and uses the machine to become a professor, Mr. Lloyd, stepping into the high school as a new teacher alongside Danny. The story blends romantic longing, comedic misadventure, and a dreamlike if reckless quest for belonging, following a boy’s quest to be with the woman he loves, and a friend’s desperate attempt to find his own place in a world that feels hostile and unkind.
