17-year-old Sydney Donovan Tia Mowry finds it is not easy adjusting to a new town in Connecticut after her family relocates from California, especially as she deals with a father’s remarriage following her mother’s death. Struggling to fit in at school and missing old friends, she also starts to navigate a crush on classmate Todd Merwin Mondesir, while Ashley Maia Campbell, his older sister, vocally distrusts and taunts her at every turn. Meanwhile, her younger brother Willie Tahj Mowry, a brilliant but restless 12-year-old, spends his days in a cluttered lab chasing bigger and bigger experiments, dreaming of defeating aging and pushing the boundaries of science.
Willie’s relentless tinkering leads to trouble when a late-night experiment spills an anti-aging formula onto a box of soap. His father Barry Phillip Jarrett quickly steps in, forbidding further experiments until Willie learns responsibility. Undeterred, Willie believes he’s close to a breakthrough, but the formula’s side effects prove more far-reaching than anyone expect. The tainted soap is used unknowingly by their Grandmother Cat Hope Clarke, and she is transformed into a 17-year-old, sparking a wild cascade of colorfully chaotic moments. Not long after, Grandpa Gene Donovan Robert Hooks follows suit, rejuvenated into his teenage self as well, introducing a pair of teenagers into a now-crowded household.
The revived Cat and Gene dive back into the thrills of youth, rediscovering the rush of first loves and old flames. Yet the experiment isn’t perfect: Willie discovers that his formula may carry deadly risks, forcing him to search for an antidote to restore his grandparents to their adult, healthy lives. As Cat and Gene navigate their second adolescence, they realize they still love each other—though Cat remains wary of Gene’s motives, haunted by the memory of him leaving for Australia two decades earlier. Their reconciliation culminates in a dance where Cat and Gene reconnect, while Sydney and Willie face their own teenage trials in parallel.
In a pivotal moment, Sydney opens up to Cat about her homesickness and the cruelty she endures from Ashley and her friends, who bully her because of Todd’s interest in her. Cat’s own history with Ashley gives Sydney perspective, showing how standing up for oneself can earn respect even from those who once resisted. At the school dance, Gene confronts Ashley in a bid to repair the past and re-enter Cat’s life genuinely. The celebration is cut short when Cat collapses from the lingering effects of the transformation, prompting Willie to sprint with the antidote. Gene carries Cat toward the pool, and in a dramatic rescue, Sydney steps up to curb Ashley’s rage by delivering a well-timed punch.
With the antidote finally in hand, Willie hurls it into the pool as Gene and Cat dive in to reverse the accidental teenage years. The bodies stabilize, and Gene musters the courage to propose to Cat again in the moment of their shared relief, and she says yes—reigniting a promise that had once been broken. Just as their happiness seems sealed, the family returns home to face the practical realities of daily life: Monique and Barry, the parents, arrive to find Cat and Gene reconciled and ready for a future together, while planning help with the wedding. Sydney and Willie must also confront a pressing new challenge: Barry’s persistence with the old soap and the risk that it remains a threat if left unchecked, prompting a coordinated effort to keep the potion and the peril away from the rest of the family. In the end, what began as a science experiment becomes a heartfelt tale about forgiveness, family bonds, and the enduring power of love across generations.
