In the midst of navigating the challenges of adulthood, Tim Templeton, portrayed by James Marsden, has carved out a cozy life with his wife Carol, played by Eva Longoria, and their two daughters: 7-year-old Tabitha and her baby sister Tina, voiced by Ariana Greenblatt. However, Tim’s life takes an unexpected turn when he discovers a shocking secret: baby Tina is a newcomer from Baby Corp, similar to her uncle Ted, voiced by Alec Baldwin, who now leads a successful career and is often out of reach.
Tim's reluctance to reach out to Ted changes when Tina cleverly leaves a deceptive voicemail for him, compelling his unexpected visit to the Templeton residence. Once there, Tim reluctantly explains to his brother that Tina is, in fact, on a covert mission from Baby Corp, which results in a much-needed reunion.
With the brothers reunited, Tina unveils a revolutionary formula that allows adults to revert to their 7-year-old selves for a limited span of 48 hours. This newfound ability becomes pivotal as they infiltrate Tabitha's school to investigate the mysterious Dr. Erwin Armstrong, voiced by Jeff Goldblum, who serves as the enigmatic principal of the Acorn Center for Advanced Childhood. Tim reverts to his youthful form, while baby Ted joins a group of infants in the playroom.
Seizing the chance, baby Ted inspires his peers to devise an escape plan aimed at gaining access to Armstrong's office, unveiling his sinister intentions. Meanwhile, Tim, now in his child form, attempts various antics to get into the principal's office but winds up in "The Box" for timeout instead. In a startling turn, Ted uncovers that Armstrong, too, is a baby with superior intellect who abandoned his family and became a successful app creator, now intent on erasing parental influence on children as part of his grand design for B-Day.
As chaos erupts during the grand holiday pageant, Tabitha, performing solo, fears the worst when Tim and Ted fail to appear. Unfortunately, their efforts to thwart Armstrong are thwarted by his latest digital menace, QT-Snap, which would control parents turning them into mindless followers. In her quest to find her siblings, Tabitha encounters Tina, who reveals her secret mission: to thwart Armstrong and aid in reuniting Tim and Ted.
Imprisoned in "The Box," which begins to fill with water, the sisters craft a bold strategy targeting the server room to disable QT-Snap before it can spread globally. During a desperate moment, Ted makes a pivotal call to Precious, facilitating his escape from The Box just in time. As the brothers reach the servers, they face off against Armstrong's devious schemes and an army of parents brainwashed by his app.
With the help of Tina and Tabitha, they ultimately outsmart their adversary by hacking the system and triggering an explosive candy lava volcano using Mentos and soda, effectively demolishing the servers and breaking the parents from their entranced states. After averting disaster, Tina’s true mission is revealed: to mend the broken bond between Tim and Ted, a goal accomplished as the Templeton family unites to celebrate Christmas. Meanwhile, Armstrong retreats into the shadows, thwarted by the cleverness and bravery of Tabitha and Tina.
