Gil Buckman (Steve Martin
) is a 35 year-old father of three living in suburban St. Louis. He works as a sales executive while his wife, Karen (Mary Steenburgen
), is a stay-at-home mom, caring for their eldest son Kevin (Jasen Fisher
), daughter Taylor (Alisan Porter
), and toddler son Justin (Zachary La Voy
). Gil is constantly struggling to find balance between his family and career, often weeding out issues with both. At work, his ethics are often overlooked and inner office politics seem to favor those with playboy personalities rather than those who work hard. At home, he tries to deal with the strains of parenthood and often sees himself as a failure of a father when he observes how unconfident his son Kevin is. The apparent issues that plague Taylor and Justin, who seems to enjoy butting things with his head, don't alleviate Gil's anxiety. He tries desperately to avoid being the type of father his old man, Frank (Jason Robards
), was; distant and callous. Karen, meanwhile, tries to remain calm and supportive.
Gil's older sister, Helen (Dianne Wiest
), is a divorced bank manager whose ex-husband wants nothing to do with her or their two children, Julie (Martha Plimpton
) and Garry (Joaquin Phoenix
). Julie is rebellious and more interested in her boyfriend, Tod (Keanu Reeves
), than she is about her high school education. They make love in Julie's bedroom, documenting it with a camera, and cause Helen strife when she accidentally picks up their developed pictures. Garry, on the other hand, is quiet and silently resentful towards the women in the house. When his father denies him the opportunity to stay with him for the summer, Garry angrily breaks into his dental office and smashes everything in sight. Helen understands his anger and tries to speak sense while dealing with her own issues stemming from a severe lack of romantic involvement. When Julie and Tod abruptly get married, Tod comes to live with the family and Helen's views of him as an unintelligent punk change when he gives consoling advice to Garry about his own budding sexuality.
Gil's other sister, Susan (Harley Jane Kozak
), is a middle-school teacher married to Nathan Huffner (Rick Moranis
), a rather neurotic scientist. Susan is distressed to see their daughter, Patty (Ivyann Schwan
), becoming a shell of a child due to Nathan's advanced home-education methods for cognitive development. She wants to have a second child but Nathan is against the idea and berates her when he discovers that she's been poking holes through her diaphragm.
One day, Gil's kid brother Larry (Tom Hulce
), surprises the family with his arrival after a prolonged absence. Never staying in one place very long and preferring to cash in on get-rich-quick schemes rather than investing in a career, Larry is the black sheep of the family but the favorite of his father, Frank. Larry introduces his son, Cool (Alex Burrall
), whose existence had been unknown to him until recently. He explains that Cool's mother was a show girl from Las Vegas who suddenly showed up with Cool, the result of their brief tryst, and told Larry to keep him while she left the country to evade law enforcement for shooting someone. Larry stays with Frank and his mother, Marylin (Eileen Ryan
), and is thrilled when Frank shows him his pride and joy, an antique Oldsmobile that he's kept in pristine condition.
When the hired cowboy intended to provide entertainment for Kevin's birthday party is accidentally swapped with a stripper, Gil resorts to ingenuity in order to avoid the inevitable meltdown should Kevin find out. He assumes the role himself, using inventive methods to turn himself into a cowboy. His entertainment proves a hit and he feels an immense swell of satisfaction and pride for giving Kevin something memorable. However, his glee is short lived when Kevin has an episode one night while out to dinner and Gil is devastated that he couldn't 'fix' his son's anxieties. Situations at work then worsen to the point where Gil quits and, when Karen reveals that she's pregnant with their fourth child, Gil finds himself at a loss and falls into a depression. He suffers nightmares where, instead of seeing his son Kevin grow up as a confident young man, he sees him becoming disturbed and suicidal; a direct result of Gil's ultimate failure as a father.
Meanwhile, sick and tired of Nathan's obsessive bearings over the family, Susan tells him, in her own way, that she is leaving him.
Helen finally seems to be fitting into a place where she is happy. She has begun dating again and goes out with Garry's science teacher, George Bowman (Paul Linke
), and Garry seems happy, spending more time with Tod. One night, after her date with George, Helen witnesses Julie storm into the house and engage in an argument with Tod after finding a drag racing helmet in his car after he'd promised her that he wouldn't race anymore. Dejected, Tod admits that he went back to racing after his friends, whom he had planned to start a small business with, took off with all of their supplies, leaving him nothing. Tod leaves the house and Julie angrily runs to her room, mentioning to the side that she is pregnant with Tod's child. Helen takes the news with a stiff drink.
When Frank discovers his antique car is missing, he begins to have doubts about Larry. Larry returns with the car, claiming that he wanted to get it appraised but Frank sees through the lie and retorts that he knows Larry was simply interested in pawning if for a quick buck. Larry confesses that he has a gambling problem and owes a hefty debt of 25K to some bookies over a basketball game. If he fails to pay, they will kill him. He begs his father to help him, reasoning that Frank should because he's his father. Humbled by his disillusions, Frank asks to meet with Gil who admits his own fears in parenting. Frank acknowledges that he wasn't a good father and tells Gil that he knows he is, but that he worries too much. Any kind of worrying is something that never goes away, even when children are full grown. Frank and Gil have a reconciliation and Frank asks for advice on what to do with Larry.
Frank agrees to help Larry and tells him that he will train him in the family business, paying the bookies over time with the money Frank intended for retirement. Larry agrees to the plan but only after he looks into a sweet opportunity in Chile and asks Frank for a grant of $3,000 for the trip. Disheartened and knowing Larry's true intentions, Frank agrees regardless. He joins Cool on the porch, who is saddened to see his father leave and asks if he will come back. Frank says that he won't but asks Cool if he'd like to stay and live with them.
Nathan surprises Susan by walking into her classroom during school hours and breaks character by serenading her with their wedding song, begging her to give him one more chance and that he can change. Susan accepts and kisses him. Her students cheer and clap.
Helen sits on the sidelines with Garry and Julie while Tod prepares his car for a race. Halfway down the track, Tod loses control and violently crashes. He is quickly loaded onto the back of a truck for evaluation, though he appears unhurt, but Julie finds the distress too overwhelming. She tells Helen that she 'can't do this anymore' but Helen reminds her that 'this is marriage' and that she needs to be with Tod. Julie embraces Tod and Garry marvels at his mother's advice. Helen tells Garry that a marriage is worth suffering through if you truly love someone but she gives her daughter's marriage a limited timeframe.
Gil and Karen help Taylor get dressed for her school play that night, though Gil is still visibly distressed. He managed to get his job back when his supervisor, Dave (Dennis Dugan
), admitted that the man promoted instead of Gil started screwing things up, but is still on edge with his family. He argues with Karen until his grandmother (Helen Shaw
) interrupts. She explains that when she was young, she went on her first roller coaster with her husband. She found that all the ups and downs, which gave her so much excitement and fear, were preferable to the merry-go-round that other people rode. Gil misses the reference to life and drives the family to the school play. Taylor has a role as a dwarf in the production of 'Snow White' and, when her character, Dopey, is shoved forward by the other children, Justin becomes defensive and runs on stage to 'save his sister', butting his head into the other children. Amid the laughs and cries of disapproval by the other parents, Gil strains against the anxiety but manages to calm himself, even finding humor in the situation. At peace, he embraces Karen and strokes her belly lovingly.
Things fall into place and, months later, the family reunites in the hospital's maternity ward to welcome the newest addition. A tearful George enters the waiting room and announces that he and Helen have a healthy baby girl. The family rejoices as Helen holds her newborn. Frank hands out cigars and holds his grandson, Cool. Julie snuggles next to Tod, holding their own baby while Gil changes his new daughter's diaper. A visibly pregnant Susan sits with Nathan while a more chipper Patty plays with Gil's three other children.