Ben Healy Jr. is a well-meaning yet beleaguered yuppie, feeling stifled by the overbearing shadow of his father, John Ritter, who goes by "Big Ben" Healy Sr. As the patriarch campaigns for mayor with an iron grip, Ben's life stagnates, particularly due to his wife Flo's struggles with fertility, leaving him feeling like a mere extension of his father's legacy. In his desperate quest for solace, he turns to the questionable adoption agency led by Igor Peabody, and what he gets is a pint-sized chaos machine named Junior, a mischievous 7-year-old.
At first glance, Junior seems innocent, but he harbors a penchant for pandemonium that disrupts the Healy household and beyond. His harmless-looking shenanigans lead to a string of catastrophes: from causing Fuzzball the cat to end up in the vet's office to making Big Ben take a humiliating fall down the stairs, and even turning his own room into a fiery mess. A family camping trip spirals into chaos when Junior decides to "enhance" the campfire by urinating on it, while also playing a prank on Roy's kids. The whirlwind of mischief continues at Lucy's birthday party, where Junior disrupts the magic show and showcases his unconventional approach to Little League—hitting opposing players with his bat.
Despite the growing doubts about whether this rambunctious child is worth the havoc he brings, Ben becomes moved when he learns that Junior has been rejected by 30 different families. This revelation triggers something deep within him, leading him to embrace the little outcast with the warmth and affection he has longed for. Unfortunately, Junior misinterprets this kindness as a cue to wreak further havoc, even robbing Ben's father, Big Ben's store, leaving the family's finances in disarray.
Things take a dramatic turn when Junior is kidnapped by a character named Beck, initially leaving Ben and Flo relieved at the temporary escape from the chaos. However, Ben’s perspective shifts when he discovers Junior’s drawings. The innocent child portrays his tormentors as monstrous figures in dark places, while depicting Ben as a gentle and kind presence in a bright world. This eye-opening moment compels Ben to recognize that Junior’s misbehavior is a guise for the pain he has endured at the hands of selfish individuals.
Driven by love and a newfound courage, Ben embarks on a daring mission to rescue Junior from Beck’s grasp. This ignites a series of bold escapades, including stealing Roy's prized vehicle and elements of his gear. When confronted by Big Ben, Ben seizes the moment to expose his father's true demeanor, broadcasting embarrassing moments to the world, showcasing the consequences of underestimating him.
The climax unfolds at the circus, where the chaotic showdown sees Junior executing a daring escape from Beck during a trapeze act. As the chase unfolds, they find themselves in a dramatic car crash that sends Flo sprawling out of a suitcase and onto a truck filled with pigs. The authorities eventually nab Beck, but not without one last shot hitting Ben in the chest.
As Junior expresses regret for not saving Ben, believing him to be dead, Ben awakens miraculously, discovering that a good-luck prune in his pocket deflected the bullet. In a heartfelt gesture, Junior discards his bow tie, symbolizing his step toward independence and distancing himself from Beck's negative influence. In this moment of triumph, Junior and Ben walk away together, strengthened by their shared struggles and newfound connection.
