Bennington Austin “Bink” Cotwell IV, the nine-month-old son of socialites Laraine and Bennington Austin “Bing” Cotwell III, lives in a mansion in a suburb of Chicago and is just about to appear in the social pages of the newspaper.
Three very clumsy criminals, Edgar “Eddie” Mauser, Norbert “Norby” LeBlaw, and Victor “Veeko” Riley, disguise themselves as baby photographers from the newspaper and kidnap Bink, demanding a ransom of $5 million. After the kidnapping, however, the criminals have difficulty controlling Bink at their apartment. Norby attempts to put him to sleep by reading his favourite storybook, Baby’s Day Out (or “Boo-Boo” as he calls it), only to fall asleep himself from boredom, leaving Bink unattended. Looking through the book, Bink notices a pigeon on the page and then one by the window; he follows it out and successfully gets away from his kidnappers. The ensuing chase culminates in Eddie falling off the building and into a garbage bin. Norby and Veeko rescue him, and they begin pursuing Bink across the city.
The FBI arrives at the mansion, headed by Dale Grissom, where they try to piece together clues along with Bink’s parents and his nanny, Gilbertine. Meanwhile, Bink, now outside on the ground and crawling about, finds another part of his book – the blue bus, which he then boards. The criminals realise he is escaping 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 woman who gets off at her stop shortly afterward. By the time the criminals catch the bus, they realize Bink is not on board and follow the lady, leading to a physical altercation after she catches them. In the distraction, Bink crawls up to a revolving door at the entrance to a department store and is forced inwards by its momentum. He is stopped by an employee who works for the store’s day care center, believing he is another baby who escaped from there. He then escapes from the store and eventually crawls into traffic after a ride on a taxi. Meanwhile, Bink’s parents receive a phone call from a concerned citizen Joe Depke who believes he saw Bink with a woman Mrs. McCray. Depke feigns memory loss when questioned until Mr. Cotwell pays him money. In the McCray’s apartment the couple realize it’s not Bink and apologize for the inconvenience. Mrs. McCray tells them she prays Bink will come back to them and knows how’d she’d feel if it were one of her kids. Just before leaving she tells Bing that she believes someone somewhere watches over the babies. The couple tell Mrs. McCray they hope she never is in their place for her children’s sake & express hope that someone does watch over the babies.
The criminals attempt to follow Bink, but keep getting injured in the process as he makes his way to the city zoo. They are shocked to find him in the ape house with a western lowland gorilla, who shows a friendly and paternal side and does not injure him. The gorilla also feeds Bink with some of his fruits. The criminals attempt to retrieve him, but the gorilla thwarts them by pounding Veeko’s hand, throwing Norby into the air using a mop as a catapult and finally grabbing Eddie close against the bars by roaring at him with intense volume and hurling him across to another cage.
The criminals corner and catch Bink in the zoo’s park, but are confronted by two chatty 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 Bink reaches his cigarette lighter, setting his groin on fire and sneaking off as soon as the officers are gone. Veeko extinguishes the fire by stomping repeatedly on Eddie’s groin.
They then follow Bink to a construction site where they experience several near-death mishaps such as Veeko getting thrown off the building and into the back of a garbage truck, Norby falling into a vat of wet cement, and Eddie getting stranded on a crane after being hit by a hammer and drenched in glue. The sun then sets as Bink and the construction crew leave the site. After managing to escape, the criminals give up on catching Bink and return home.
Bink’s parents are notified of various sightings of him in the city and Gilbertine deduces that he has been following Baby’s Day Out, and will most likely head for the Old Soldiers’ Home next. Sure enough, Bink has made his way inside the home, where the elderly residents entertain him with a rendition of Irving Berlin’s “This Is the Army, Mr. Jones.” Laraine and Bing run in and joyously embrace Bink. On the way home, he begins to call out for his “Boo-Boo” towards the criminals’ flat. Laraine believes he is looking at the clock on top of the building and tries teaching him it’s a “tick tock”. Gilbertine again deduces he wants his book and tells Laraine there’s no “tick tock” in the book. The recuperating criminals, upon hearing Bink calling out for his book, realize that he has returned, and upon looking out the window, to their shock, they find themselves surrounded by the FBI. They also find Bink and his parents standing outside the building as well. As Eddie berates Bink for ratting them out, Grissom forces the criminals to return Bink’s book. As Eddie throws down the book, he, Norby and Veeko are arrested for kidnapping Bink and Bink is happy to have his book back and returns home with his parents.
Back at home, Bink is put to bed by his parents, who discuss having his photograph taken by a normal photographer in the morning while, unbeknownst to them, he wakes up and gets ready to read another book titled Baby’s Trip to China.
