Bennington Austin "Bink" Cotwell Jr. (Adam and Jacob Worton), a mischievous baby who lives in a huge mansion in a suburb of Chicago with his parents Laraine (Lara Flynn Boyle) and Bennington Sr. (Matthew Glave), is just about to appear in the social pages of the newspaper. Three klutzy would-be kidnappers: Edgar "Eddie" Mauser (Joe Mantegna), Norbert "Norby" LeBlaw (Joe Pantoliano), and Victor "Veeko" Riley (Brian Haley) disguise themselves as the photographers from the paper and kidnap him. After the kidnapping they have difficulty controlling him. While trying to get Bink to fall asleep, Norby does so reading Bink's book titled "Baby's Day Out", leaving him unattended. Looking through it, Bink notices a bird on the page and then one by the open window. He follows it out and successfully gets away from his kidnappers, with Eddie falling off the building and into a garbage bin while chasing after him through the rooftop.The FBI arrives at the mansion, headed by Dale Grissom (Fred Thompson), where they try to piece together clues along with Bink's parents and his loving nanny Gilbertine (Cynthia Nixon). Meanwhile, he, now outside on the ground and crawling about, finds another part of his book: The blue bus, which he then boards. The kidnappers realize he is missing and start chasing the bus in their van but their efforts are in vain. Meanwhile on the bus Bink crawls into the bag of an obese lady who gets off at her stop shortly afterwards. By the time the trio catches it, and realize Bink is not on board, they then realize that he crawled into the lady's bag and follow her. An altercation ensues after they insult her, and while they attempt their escape, Bink crawls up to a revolving door at the entrance to a department store (also featured in his book) and is forced inwards by its momentum.Crawling through the department store, Bink is stopped by a worker (Dawn Maxey) who works for Mother Goose Corner, a nearby day care center, who believes he escaped from there. While unattended there, he crawls into the bottom of a stroller and is wheeled out by an unsuspecting mother, leaving the store, he crawls out from the stroller and eventually ends up in traffic. The kidnappers attempt to follow him but get their feet run over by a passing truck and jump into a giant ditch as he makes his way to the city zoo. They find him in the primate house with a gorilla, and lose hope of gaining their ransom money. The gorilla shows a maternal side and does not injure him. The kidnappers try to retrieve Baby Bink but she notices them and bashes Veeko's hand, throws Norby into the air using a mop-stick as a catapult, and hurls Eddie against the bars of the cage opposite her own.The kidnappers eventually corner and catch Bink in the zoo's park, but are confronted by two friendly police officers, who have noticed that their van's engine is still running. During the conversation, Eddie hides Bink under his coat in his lap, but he reaches Eddie's cigarette lighter, sets his crotch on fire, and sneaks off as soon as the officers are gone. Veeko extinguishes the fire by repeatedly stomping on Eddie's groin, seriously hurting him. They follow Bink to a construction yard, but are still unable to catch him, with Norby falling into a vat of wet cement, Veeko getting thrown off the building and into the back of a garbage truck, and Eddie getting stranded on a crane after being drenched in glue. The sun sets as Bink leaves the construction yard. The kidnappers manage to escape (offscreen), but decide to give up and go home.Bink's parents are notified of various sightings of him in the city and Gilbertine deduces that he has been following the happenings of his favorite book, (or "Boo-Boo", as Bink calls it) and will most likely head for the Old Soldiers' Home next. Sure enough, they find him there, but on the way home, he begins to call out "Boo-Boo" toward the kidnappers' flat. The FBI moves in on there and arrests the three villains demanding that they return his book first. Back home, Bink is put to bed by his family. As his parents discuss having his picture taken by a normal photographer in the morning, he wakes up and gets ready to read another book, this one entitled Baby's Trip to China.