Bennington Austin "Bink" Cotwell IV, the nine-month-old son of socialites Laraine and Bing Cotwell, Lara Flynn Boyle and Matthew Glave, Adam Robert Worton, lives in a grand mansion on the edge of a Chicago suburb and is about to appear in the society pages. His peaceful life is about to be upended when three bumbling criminals enter his world in a daring plan that goes dangerously awry.
Three criminals—Eddie Mauser, Norbert "Norby" LeBlaw, and Victor "Veeko" Riley—Joe Mantegna, Joe Pantoliano, and Brian Haley respectively—pose as baby photographers from the newspaper and kidnap Bink, demanding a ransom of $5 million. The trio struggles with the infant after the kidnapping, a chaotic scene that highlights their ineptitude as they try to control him in their apartment. Norby attempts to lull the baby to sleep by reading his favorite storybook, Baby's Day Out (which the boy affectionately calls Boo-Boo), but Norby himself nods off, leaving Bink unattended. In a small moment of luck, Bink notices a pigeon on the page and spots another one outside the window; he follows the birds, seizing the chance to slip away from his captors. Eddie, distracted and surprised, tumbles from the building into a garbage bin, and Norby and Veeko scramble to recover him as they chase Bink through the city.
The FBI arrives at the Cotwell mansion, led by FBI Agent Grissom, as the family and their nanny, Gilbertine, try to piece together what happened and where the child might be headed. Gilbertine, played by Cynthia Nixon, provides calm guidance amid the escalating chaos as the law closes in. Meanwhile, Bink, now crawling on the ground, discovers another part of his book—the blue bus—and crawls right onto it. Realizing their target is slipping away, the criminals pursue the moving bus in their van, but their efforts are in vain as the child continues his unplanned adventure.
On the bus, Bink squeezes into the bag of an obese passenger who gets off at her stop. By the time the criminals catch up to the bus, they realize Bink is no longer aboard and head after the woman, clashing with her in a tense altercation. A revolving department-store door becomes a new obstacle as Bink is swept inside, mistaken by a store employee for a child who had escaped from the store’s day-care center. He escapes again, eventually finding his way into traffic after riding in a taxi.
Back home, Bing and Laraine receive a stream of reports about sightings of Bink. Mrs. McCray, a kind neighbor, provides a moment of warmth as she prays for all children to be safely returned. Mrs. McCray’s apartment scene leads to a quiet, reflective exchange about the idea that someone somewhere is watching over the babies. The pursuing criminals continue to slip and stumble, sustaining a string of near-death mishaps: Veeko is hurled from a high ledge into a garbage truck, Norby tumbles into a vat of wet cement, and Eddie is trapped on a crane after a hammer strike, then splashed with glue.
The chase finally pushes toward a dramatic rooftop encounter, culminating in a confrontation with two chatty police officers who notice the engine of the criminals’ van still running. Eddie attempts to hide Bink under his coat, but the baby uses a cigarette lighter to set his own groin on fire, slipping away once the danger subsides. Veeko stamps out the flames, and the trio presses on toward a construction site that becomes their next obstacle course. The day’s sun sets on the site as Bink and a crew of construction workers slip away, leaving the criminals to retreat home in defeat.
Gilbertine deduces that Bink has been following the adventures in Baby's Day Out and will likely head for the Old Soldiers' Home next. Sure enough, Bink makes his way inside the veterans’ home, where retired servicemen seated in front of the television cheer as he wanders in. They greet him with a lively performance of Irving Berlin’s "This Is the Army, Mr. Jones," a moment that fills Laraine and Bing with joy as they rush to embrace their son. The boy’s journey has drawn to a close as Laraine and Bing stand outside the building and call to him, while Bink calls for his Boo-Boo, knowing the familiar book still sits in the criminals’ apartment.
When Eddie, Norby, and Veeko hear Bink’s voice from the street, they realize their custody has ended in a humiliating failure. Grissom and the FBI close in, arresting the trio and retrieving Bink’s beloved book, finally returning it to his relieved parents. Back at home, Bink is tucked into bed by his grateful parents, who plan to have his photo taken by a regular photographer in the morning. Unbeknownst to them, the clever infant wakes during the night, picks up another book—Baby's Trip to China—and tilts his head toward the next adventure, suggesting that this tiny explorer’s journey may be far from over.
