14 Going on 30

Danny O'Neil is secretly in love with his teacher, Peggy Noble, who is engaged to Roy Kelton. To win her, he becomes obsessed with turning himself into a grown man using the same machine Lloyd Duffy is tinkering with. He imagines that adulthood will give him the power to separate Peggy from Kelton and start a romance.

That very night, Danny sneaks into Lloyd's lab and activates the growth accelerator, transforming into a 30-year-old man. He revels in his new freedom and the attention it brings. He disregards the potential consequences of his reckless plan.

The following day, Lloyd begins work on a device to reverse the aging and restore Danny to his fourteen-year-old self. His prototypes fail, and his test subjects (tomatoes) die, underscoring the dangers of messing with age.

At the high school, a delayed arrival of the real Harold Forndexter and miscommunication lead staff and students to assume Danny is the new principal. Danny steps into the role with awkward ease.

As Harold, he unveils rules that stress fun and a carefree atmosphere, which charms Peggy and unsettles Kelton. The students respond to this lighter approach with enthusiasm.

Peggy, influenced by Harold's youthful approach, agrees to go on a date with him, despite Kelton's attempts to keep them apart. Kelton shadows their movements, growing suspicious.

Danny and Peggy share a splendid evening, and the chemistry between them deepens. Peggy begins to see Harold as someone she could be with, nearly sharing a kiss by the night’s end.

During the school dance, Danny convinces Peggy that Kelton is not right for her, and she breaks off the engagement. Kelton, furious, begins digging for the truth about Harold.

Following the courtship, Danny and Peggy officially become a couple. Peggy’s feelings have grown beyond the deception, even as Harold’s identity remains a secret.

Lloyd finally completes a working reversal device, prepared to restore Danny to his true age. When he informs Danny about it, Harold’s persona has taken over, and Danny refuses to revert back.

Kelton discovers Harold is an impostor and promptly informs the police, who arrest him. Danny is then forced to transform back into his fourteen-year-old self.

Danny tries to slip away as officers close in, while Lloyd prepares the machine to facilitate his return to youth. After the confrontation, Harold tells Peggy that he must leave town for good.

Peggy follows Harold back to Lloyd's lab and witnesses the transformation back into Danny, realizing Harold was one of her favorite students. Her perception shifts as she recognizes real feelings.

Moved by love, Peggy asks Lloyd to turn her into a fourteen-year-old so she can be with Danny, stepping briefly into his world. The transformation is a bold choice meant to seal their bond.

Meanwhile, in a twist of fate, Lloyd gives in to temptation and uses the machine to become a professor, Mr. Lloyd, and begins teaching at the same high school. The school world shifts as both a grown teacher and a young couple navigate their new dynamics.