A woman delivers an infant to a mansion, rings the doorbell, and hurries away. When the lady of the house picks up the baby boy, he urinates in her face. The maid leaves the basket at another house. Several years later, a woman drops the little boy off at another house. He poisons the cat and sucks a goldfish out of its tank with a vacuum cleaner. Abandoned at a trailer park, he becomes angry over a broken toy and demolishes his new home with a bulldozer. Taken to a Catholic orphanage called St. Brutus, the boy, now called “Junior,” devises booby traps for nuns. Meanwhile, in the town of Cold Water, a doctor informs Ben and Florence “Flo” Healy that she is infertile. Back at the orphanage, Junior continues wreaking havoc with the nuns and a priest. Watching “Bow-tie Killer” Martin Beck being recaptured on television after a prison break, he sympathizes with the criminal’s screams of being “misunderstood.” Junior takes a bow tie off a doll and wears it constantly, just like Martin Beck. Meanwhile, Ben Healy arrives at his job at Big Ben’s sporting goods store, owned by his father, “Big Ben” Healy, Sr. He observes a young boy and his father having a close rapport, and the father rhapsodizes to Ben that having a son “makes life worth living.” Ben is soon informed that his own father, Big Ben, is selling the store to the Hirohito Corporation and leaving him nothing, despite Ben’s ten years of working there without a raise or a promotion. Big Ben tells him, “Don’t trust anyone, especially your own father.” Making matters worse, neighbor Roy, who coaches a Little League baseball team that always beats the team Ben coaches, joyfully shows Ben a sonogram of his soon-to-be-born son. At St. Brutus, several nuns display Junior’s disturbing drawings of skeletons, monsters, and headless bodies to Mr. Peabody, the orphanage psychologist. When Mr. Peabody brings the boy into his office, Junior realizes that the only way to get out of the orphanage is to make the psychologist hate him as much as the nuns do, so he insults Mr. Peabody with profanity. In class, while other children craft letters to Great Britain’s Queen Elizabeth and South Africa’s Bishop Tutu, Junior writes to Martin Beck. He tells the convict he wears a bow tie just like him, and brags that he is “getting out” soon. Meanwhile, Ben Healy wishes for a boy of his own. He convinces Flo that a child would be a wonderful fashion accessory for her, as well as an entrée to all the children’s parties in their upscale neighborhood. When Ben and Flo visit St. Brutus, Mr. Peabody promises to accelerate the normal seven-year wait by providing them with a seven-year old boy named Junior. Flo accepts the arrangement because she can skip the diaper years and immediately attend neighborhood children's parties. The Healys pick up Junior at the orphanage, and the moment they leave, nuns lock the gates and celebrate. Arriving at his new home, Junior knocks everything off the living room mantel, then frightens Fluffball the cat and Polly the parrot. Ben and Flo invite Big Ben to the house for a surprise, but he is outraged that they have adopted a child who may be genetically inferior. When Junior sets his own bedroom on fire because he hates the clown decorations, Big Ben declares him a “bad seed,” prompting Junior to throw the cat at him, propelling the elder Healy down the stairs. As ambulance attendants wheel Big Ben out of the house, Junior steals his wallet. Meanwhile, during a psychiatric test, Martin Beck strangles Dr. Rupert Camarillo, puts on the doctor's clothes, and leaves the prison in his car. Late at night, Ben Healy finds Junior wandering around the house and serves him milk in the kitchen. When Junior laments that his new parents will soon want to get rid of him, good-hearted Ben swears that they will be together forever, because he is determined to be a good dad. The next day, neighbor Mrs. Henderson and little daughter, Lucy, visit. Lucy does not want Junior at her sixth birthday party, but after pressure from her mother, she grudgingly invites him. Ben takes Junior camping with Roy’s family. Angry at being ignored by Roy’s six children, Junior urinates on the campfire and uses a pork chop to lure a bear into camp. Ben thinks the bear is merely Roy in a costume, and changes his mind only after it throws him onto a tent. When Roy arrives moments later in a bear costume, Ben mistakes him for the real bear and knocks him unconscious with a skillet. On Saturday, Junior attends Lucy Henderson’s party wearing a devil costume. The other children reject Junior because “he’s not even a real kid, he’s adopted,” and Lucy refuses to let him watch her magic show. Ben placates Junior by giving him a petrified prune his own grandfather bequeathed him as a boy, and makes Junior promise he will take care of it as a symbol of the bond between them. Angry at Lucy, Junior bursts all the balloons, drags a lawn sprinkler into Lucy’s bedroom, and throws her birthday presents into the swimming pool. He also puts firecrackers in the birthday cake and drops a frog into the punch bowl. That night, Flo demands Ben punish the boy for ruining Lucy’s party, but when he goes to Junior’s bedroom, he merely tells him to contemplate his misdeeds. Meanwhile, Martin Beck heads for Cold River, where, on “Founder’s Day,” Big Ben Healy is being feted as a candidate for city mayor. Big Ben gives a speech at the baseball field, where Ben and Roy are coaching opposing teams. Ben puts Junior in uniform, despite the boy’s protests that he knows nothing about baseball. When he goes to bat, Junior swings at a pitch, loses his grip, and sends the bat flying into Big Ben’s windshield. As the boys on the other team chant insults, Junior gets a hit and carries the bat around the bases, knocking down opposing players as he runs to home base. Ben takes Junior to the local church to talk with the priest, but Junior hides in the priest’s side of the confession booth. Ben steps in to confess for his “problem” child and lament how everyone hates the boy. Junior listens, but when Ben wonders if he should return the boy to the orphanage, Junior says, “No, it’s the worst thing you can do.” However, Ben has already stepped out of the booth and does not hear Junior’s words. When Mr. Peabody admits to Ben and Flo that Junior was returned thirty times before they adopted him, Ben defends the boy and declares that they are going to love him even when he is bad. Outside, Junior steals Ben’s car, despite being unable to see over the steering wheel. With Ben hanging onto the hood, Junior crashes into Big Ben’s store. Later, Ben receives a telephone call informing him that his bank account has been emptied by Big Ben to pay for the damage. In shock from the accident and the loss of his money, Ben goes to Junior’s room holding a “Home Sweet Home” pillow to suffocate him, but suddenly Martin Beck arrives. Thinking he has found a like-minded pal, Beck is angry to discover that “J. R.” is a seven-year-old boy. Assuring the escaped killer that he is turning eight in two weeks, Junior introduces him to Ben and Flo as “Uncle Marty,” and Flo is excited that a relative has claimed the boy. She invites Beck to spend the night, feeds him, and raves about what a great boy Junior is. When Beck kisses her in the kitchen, Flo sexually succumbs. The next morning, Uncle Marty leaves a ransom note on the wall demanding $100,000 if Ben wants to see Flo and Junior alive again, but Ben is still too dazed from the accident to respond. Seeing Beck’s photograph in the morning paper under the headline “Bow Tie Killer Manhunt,” Ben is happy that Flo and Junior have gone. Meanwhile, in Beck’s car, Junior is unhappy that Beck dislikes him. Flo is tied up in the back seat, angry because she thought she and Beck were going on a sexual adventure. Beck stuffs her in a suitcase and puts her in the trunk. At home, as Ben throws Junior’s toys out the window, he sees a pleasing drawing of “Mr. Healy” and realizes his son actually likes him. He drives to the store, where Big Ben is preparing to make a television appearance. Ben asks for $100,000, but his dad tells him to get lost. Flipping on the television “feed,” Ben allows the TV audience to watch Big Ben brag about selling out to the Japanese and screwing the people of Cold Water. Ben drives home in time to receive Beck’s telephone call ordering him to bring money to the tiger cage at the circus. Ben steals Roy’s station wagon and confronts the killer. Junior kicks Beck in the groin, escapes, and runs into the circus tent. As he climbs among the trapeze artists, Beck follows, but Junior loosens a line and drops him to ground. Beck gets into his car and escapes, with Ben and Junior pursuing in Roy’s car. A bump sends the suitcase containing Flo onto the roof of Roy’s car. Using Roy’s rifle, Junior shoots out one of Beck’s tires, causing him to crash. As Ben rear-ends Martin Beck, the luggage flies off the roof and into the bed of a passing truck. Beck fires a gun at Ben and hits him. Thinking his dad is dead, Junior cries for Ben to wake up. He revives unharmed, because his father’s campaign button and the fossilized prune in his pocket stopped the bullet. The boy gets rid of his bow tie. As police haul Martin Beck away, Junior wonders why Ben loves him, but is willing to love him back. Meanwhile, in the back of a truck, Flo opens the suitcase and finds herself staring into the rear end of a flatulent pig.