When 12-year-old Willie Donovan's home science project goes awry, his estranged grandparents magically turn 17 again. But they won't enjoy their long-lost youth for long unless Willie can find an antidote to prevent them from dying prematurely.
The Donovan family has just moved from Los Angeles to a small town in Connecticut. Willie has adapted well and spends a lot of time in the garage where he is experimenting with a secret formula to arrest the aging process. His sister Sydney is less than thrilled at having been uprooted from her cool life in California. Soon their antagonistic grandparents, Gene and Cat, who have been divorced for 25 years, arrive for a visit. When Willie and Sydney's parents have to leave town unexpectedly for a few days, they are left in the care of their bickering grandparents.
Unbeknownst to Willie, a bar of soap has been contaminated by a tiny drop of his experimental formula. Later, Gene takes a shower and Cat has a bath. Much to their amazement, they suddenly turn into youthful 17-year-olds. After the initial shock, young Gene revels in his newfound youth. Young Cat, meanwhile, looks enough like Sydney to be her twin sister and becomes her new best friend.
When the family dog also becomes part of the experiment, Willie soon discovers that the canine is slowly dying. He must work quickly to find an antidote before his grandparents suffer the same fate. On a positive note, being young has rekindled Gene and Cat's love for each other. But it becomes a race against time as Willie struggles to reverse his experiment.