Plot
In the 1970s, Tim Leslie Templeton, a creative 7½-year-old boy, is taken aback when his new baby brother, The Boss Baby, arrives. Baby wears a suit and tie and acts like a normal baby around parents and adults, but walks and talks like an adult when parents are absent. Tim catches him one night and Baby tells him that he needs to make way for the next generation and stay out of his way. The next day, Baby holds a staff meeting with five other infants, under the guise of a neighborhood play date. Tim attempts to record them on a tape before Baby and his cronies spot and chase him, resulting in it being destroyed. With no evidence to support him, Tim is grounded for trying to get rid of Baby and is ordered to stay in the house with him until he learns to get along, much to both of their dismay.
Later, Baby reveals the truth as to why he is in his house and where he comes from. He and Tim suck a special pacifier that allows them to see Baby Corp, where babies come from. Most babies go to families, but those unresponsive to tickling are sent to management, where they are given a special baby formula that allows them to think and behave like adults while remaining young forever. Baby explains he is on a special mission to investigate the declining love for babies due to puppies, which is why he came to the Templetons as Tim's parents work for Puppy Co, and he will leave once his mission is done. Once he does, he will get a promotion to CEO. However, the boys hear Baby's boss threatening to fire him if he fails, which would mean Baby would have to stay, have his formula taken away, slowly turn into a normal baby, and grow up with the Templeton family. Tim and Baby team up to prevent this.
On Take Your Kids to Work Day, the parents lift the grounding when the boys pretend to love each other and take Tim and Baby with them to Puppy Co. While investigating, they are caught by Puppy Co.'s CEO, Francis E. Francis, who is then revealed to be Baby's longtime idol: Super Colossal Big Fat Boss Baby, a former CEO of Baby Corp who got fired due to aging from lactose intolerance. They sent him to live with a family, feeling betrayed. He takes away Baby's formula to create a "Forever Puppy", a puppy incapable of aging, which will take all love from babies and give him his revenge on Baby Corp.
Francis takes Tim's parents to a Las Vegas conference and leaves his older brother Eugene to pose as a female nanny to watch them. Without his formula, Baby begins to have episodes of being a normal baby. The boys attack Eugene with fake vomit and escape from him with the help of the neighborhood toddlers. They reach Las Vegas, where they find Francis ready to launch a rocket of Forever Puppies out into the world. Tim's parents are trapped below the rocket to be burned. Tim and Baby fight Francis on a catwalk, making him fall into a vat of formula that turns him back into a baby. Tim and Baby save Tim's parents and eject the Forever Puppies from the rocket before it launches. With his plan foiled, Francis tries to take them on in his current stage. Eugene picks him up and speaks (for the only time in the movie), telling the boys, "This time, we'll raise him right." before leaving.
With his mission a success, Baby goes back to Baby Corp and becomes CEO. Baby Corp workers erase evidence of Baby and the parents' memories of him. One of these workers asks Tim if he would like to forget about Baby, but he declines. Tim and Baby soon realize they miss each other deeply, and Tim invites him back, saying that he wants him back as his brother. Baby returns as a regular baby named Theodore "Ted" Lindsey Templeton, realizing love is something that grows, instead of being divided.
Years later, in the 2000s, an adult Tim and Ted tell the story to Tim's daughter, Tabitha, who is apprehensive about the arrival of her newborn baby sister. The newborn girl, named Tina, is revealed to be an employee of Baby Corp, like her uncle, surprising Tabitha.
