Mollie (Kirstie Alley) and James (John Travolta) are married. They are teaching their baby Mikey (Lorne Sussman) on how to use the potty. Meanwhile, James is working diligently to bring in more income for the family.The movie opens with Mollie and James preparing to have sex, with him ensuring her. Mikey gets scared and calls James into the room. He goes to check on him and returns to Mollie. Although she fell asleep, he gets her back in the mood, and they continue. The film cuts to his sperm cells traveling toward her egg; however, it teases them since the diaphragm is blocking them from getting it. One manages to get around it, and a baby girl is conceived.They tell Mikey that he's got to be potty-trained. Mollie discovers she's pregnant. Mikey learns that with his new baby sister on the way, he has to be a responsible big brother. When she is about to be born, her umbilical cord gets caught around her neck, putting her in distress. She is born through a C-section and is taken to the nursery. She is named Julie (Megan Milner).When she and Mikey meet, he says hello, and she asks, "Who's this a**hole?" He gets jealous of her immediately. One day, Mikey and his playmate, Eddie, try to walk off with her stroller and decide to ditch her at the zoo. Mollie stops the boys and when she does, she catches them with a crack pipe (which greatly disgusts and frightens her).Meanwhile, Mollie's slacker younger brother, Stuart (Elias Koteas) comes to stay, to whom James takes an immediate dislike. Mollie and James fight about her brother Stuart staying there which leads to her kicking James out. Stuart soon develops a romance with her best friend Rona (Twink Caplan), and they even start dating.Mikey is upset about James being gone and out of anger, he tears up one of Julie's favorite stuffed penguins (which she calls "her friend, Herbert"). James occasionally hangs out with his kids (including scamming their way into a movie theater) and has fun with them. Meanwhile, Mikey is having trouble using a potty, so he gets some advice from Eddie (Danny Pringle), who's already potty trained. However, he warns him about Mr. Toilet Man, who's actually just a figment of their imaginations.One day, Julie sees Herbert's head and decides to learn to walk and leave. She manages to walk without support. Mollie sees this, gets excited, and calls James and Stuart. As he watches Julie sleep one night, Mikey realizes how badly he's treated her and resolves to change his ways. Mollie realizes that she needs James to help her. She dresses up in an attempt to get back together with him. A few days later, Mikey uses the toilet and calls his parents, who are immensely proud of him. Mollie exclaims that she hasn't felt that happy about him since he was born.One night as James prepares to fly, Mollie watches the news and learns that storms are all around the area. She goes to get James before he takes off, leaving Stuart with Mikey and Julie. She catches him and tries to persuade him not to take off, just as the control tower cancels the flight. They then make up.Meanwhile, a burglar (presumably the same one who also robbed Rona) breaks in and runs when Stuart comes in with his unloaded gun. Stuart pursues him having forgotten about the kids and completely oblivious to the fact that he left paper on a hot stove which quickly causes a fire to start. Mikey doesn't panic and takes charge, pushing Julie out of the apartment to safety.Stuart and the burglar run into James who subdues the thief. Mollie and James soon find out the kids were left alone and spot the fire in the apartment, only for Mikey and Julie to emerge from the elevator as the two prepare to head in to save them. James then puts out the fire before it can cause too much damage.The next day, James, Mollie, Stuart, Rona and Mollie's parents attend a barbecue. There, Julie asks Mikey why he saved her when they're always fighting. Mikey tells her that for as much as they get on each other's nerves, they're the kids and should stick together since the grown-ups never make any sense to them. The two then walk off hand in hand.