Plot

A young boy just short of a monster is adopted by a loving man and his wacky wife. The laughs keep coming as he pushes them to the limits.

Ben is a good-hearted guy who's always wanted a son of his own, but so far he and his wife have had no success. The couple adopt Junior, a redheaded hellion who quickly makes their lives miserable. Disaster follows Junior wherever he goes, from camping trips that require emergency-room visits to birthday parties that turn into Grand Guignol affairs. When Ben's incensed family tells him to get rid of the boy, he starts to wonder if they might be right.—Official synopsis

More than anything, Ben and Flo want to take their relationship to the next level and start a family. However, they are running out of time and options. Suddenly, adoption sounds fine; after all, adopting an older child means they will spare themselves the trouble of changing diapers. That's when Junior enters their life--a charming 7-year-old orphan with the face of an angel and a knack for causing chaos wherever he goes. As the misunderstood troublemaker rains on Ben and Flo's parade, the question looms large in everyone's mind. What is wrong with the Problem Child?—Nick Riganas

Set in the suburban community of Cold River, infant Junior is abandoned by his parents and is continuously not accepted by any others, because of his increasing streak of bad behavior. At age seven, he is sent to live in an orphanage, where he continues his terror on the nuns and other kids. He also idolizes notorious killer Martin Beck, after seeing him on the local news being sent to prison for his latest crime. At the same time, Ben and Flo Healy, Ben and Flo, are upset that they cannot produce a child because of Flo's uterus and she refuses to even think about adoption. Ben also works for his father, Big Ben Healy, the owner of a sporting goods superstore and also as a potential candidate for town mayor. One day, he announces to Ben that he is selling the store to a Japanese conglomerate, as well as refusing him the land the store was built on, despite him working in the store for ten years without raises or promotions. Meanwhile, the conflict between the nuns and Junior comes to a boiling point and the nuns warn adoption agent Igor Peabody they will walk out unless Junior is released from the orphanage, much to his delight. Meanwhile, Ben manages to persuade Flo they should adopt Junior and they take him home. They later invite Big Ben over to meet Junior, but is mortified to learn Ben and Flo chose adoption. When they discover Junior's new bedroom set on fire, Big Ben already thinks Junior is already a bad seed. As the adults leave the room, Junior throws Fuzzball, the cat, onto Big Ben, causing him to fall down the stairs. He is subsequently taken away in an ambulance while Fuzzball suffers a pair of broken legs.

Meanwhile, in prison, Beck is undergoing a psychological examination. After the warden impedes one too many times, the doctor ushers him out and, shortly after, while looking at an inkblot, Beck snaps, strangles the doctor, and escapes by stealing his clothes, car, stuffing his dead body in the trunk, and posing to a gate guard as he drives away. That night, Junior is caught by Ben in the living room. They have a heart-to-heart in the kitchen and both admit that they are scared being a family, but are determined to make it work. The next day, Ben and Junior are invited to a weekend camping trip with another family, the Whites, led by father Roy, with his kids immediately rejecting Junior. That night, Junior lures a grizzly bear into their camping site, with a raw steak, to scare Roy's kids, not knowing that he also dresses up like one to playfully scare them. Hearing the commotion, Ben, wielding a frying pan, smashes Roy on the head several times. He takes off the bear's head and falls on the ground unconscious, while a baffled Ben looks up to see Junior sitting in a tree, laughing gleefully.

When the Healys are later invited to a birthday party, Junior gets revenge on the spoiled birthday girl for not inviting him to the magic show by throwing all the presents in the pool, replacing the pinata candy with canned peppers, putting a frog into the punch bowl, and replacing the birthday cake's candles with firecrackers. That night, Ben punishes Junior by taking away his allowance, but seems to have no effect as he turns an annual Founder's day baseball game, also as a mayoral campaign for Big Ben, into a baseball bat melee with the opposing players. Fed up, Ben and Flo decide to take him back to the orphanage, but when Peabody explains how many times he has been returned by other parents, Ben is determined to keep him and love him, something no other parent has done, but Junior still gets the impression they do not want him and drives her car into the sporting goods store. Later, Ben receives a harrowing phone call from the bank stating that his entire live savings was wiped out by Big Ben to pay for the store's damages.

A crazed Ben attempts to smother a napping Junior with a pillow until Beck shows up, claiming he is Junior's uncle. Flo thinks they should play along into thinking Junior is a good kid so they can get rid of him. However, the next day, Ben discovers that Beck has kidnapped both of them and is given a short time to pay a $100,000 ransom. He speeds over to the store, where Big Ben is making a live television appearance for the election. He refuses to give Ben the money, stating that the kidnapping was the best thing that happened. In retaliation, Ben turns on the camera monitor as Big Ben continues to ramble on, not realizing he is live, about how he does not care about the voters and moons the camera, causing him to be subsequently disqualified from the mayoral position. With the money now in a duffel bag, Ben returns home to steal Roy's Jeep and races to a local circus where Beck is waiting, but he double-crosses Ben by deciding to keep both the money and Junior, but he kicks Beck in the crotch and runs off with him in pursuit. He follows Junior onto a trapeze catwalk in one of the circus tents, but disconnects a holding bar, sending Beck through a trampoline. Ben and Junior then chase after Beck getting away in his car.

During the pursuit, a bump launches Flo, stuffed in a suitcase, onto the roof of the Jeep. Ben uses Roy's shotgun to shoot out one of Beck's rear tires, causing him to overturn his car on a bridge. When Ben and Junior slam into the car, Flo is launched in the air and below onto the back of a passing farm truck. When the police arrive, Beck immediately gets into a scuffle with the arresting officers and shoots Ben with one of their guns. Thinking Ben has perished, Junior tearfully apologizes for his wrongdoings and promises that he will be good. However, Ben regains consciousness and discovers the bullet went through a friendship prune his grandfather gave him and passed it onto Junior, who secretly kept it in a drawer in his bedroom. As Beck is hauled away by the cops, Ben and Junior happily walk away and, on the farm truck, Flo unzips the suitcase to see the backside of a flatulent hog, much to her disgust.