James and Mollie Ubriacco begin to make love, but Mikey, their young son, screams as he imagines a devil possessing his toys. James goes to calm the little boy, and when he returns, Mollie is asleep. James awakens her, and during lovemaking, one of his sperm “tadpoles” slips past Mollie’s diaphragm cup, swims to an egg, and fertilizes it. Later, Mollie Ubriacco brings Mikey a “potty” to begin toilet training, and the boy is intimidated. Rona, Mollie’s best friend and fellow accountant, explains during lunch how food choices can determine the gender of a developing fetus. When Mollie takes Mikey to the office where she works, her boss, Mr. Ross, objects, and the boy pours a bottle of “White-Out” liquid paper eraser on his shoe. As Mollie’s well-to-do Jewish parents, Rosie and Lou, work on Mollie and James’s joint income tax return, Rosie belittles James’s earning capacity as a taxicab driver. James tries to regain his dignity by reminding Rosie that he earns extra money as a free-lance flying instructor. Mollie lays Mikey’s hand on her stomach to let him feel the baby, and informs him that he will soon have a little sister. Mikey imagines walking the baby in a stroller on the streets of New York City and protecting her from bad boys. During a meeting with tax clients, Mollie goes into labor and is whisked to the hospital. James arrives in time to witness the Caesarian birth and passes out at the sight of blood. After James introduces Mikey to his baby sister, Julie, the boy tells the infant he will be in charge for the next few years. At the park, Mikey and his little African-American friend, Eddie, discuss making the big change from wearing diapers to sitting on a potty. Alarmed by Mikey picking a crack pipe off the ground, Mollie decides the park is too dangerous. Over James’s objection, she enrolls Mikey in “Baby Gym.” The couple argues over Mikey’s bedtime, because James wants to watch television with his son, while Mollie wants the child to get plenty of sleep. James insists that Mollie’s larger income as an accountant does not give her the right to make all household decisions. Lou, Mollie Ubriacco’s father, arranges for James to be interviewed for a pilot’s job at Airborne Express, but James feels her parents are trying to control his life. He reluctantly goes to the interview and gets hired. Stuart, Mollie’s obnoxious brother, arrives at the apartment because he lost his corporate accounting position and has nowhere to stay. Despite James’s objection, Mollie lets Stuart sleep on the living room couch. Mikey wakes up at night with a strong need to urinate, but is afraid the toilet’s mouth will bite his bottom. Mikey teaches Julie which toys she must avoid, because they belong to him. James complains to Mollie that corporate clients he flies in Airborne Express’s jets treat him as if he were still a cabbie. Also, he must carry a beeper and be on call twenty-four hours a day. One night, as James comes home late, Stuart pulls a gun on him in the dark. Stuart claims he needs the gun for emergencies, but James does not want it in the house with two young children. At the Baby Gym, Joel, the owner, leads children in exercises, and Mikey urinates in his pants. At home, James resents Stuart wearing his clothes, borrowing money from Mollie, and lying around the apartment all day. When Mollie refuses to evict her brother, James packs his clothes and leaves instead. Mollie wallows in loneliness and cries as she watches people kiss on television. Her friend, Rona, asks to stay temporarily with Mollie because her apartment was burglarized and she is afraid to go home. Rona immediately falls in love with Stuart. When Mikey tears up his sister’s favorite stuffed toy, Julie decides to leave home as soon as she learns to walk. James picks up Mikey and Julie, takes them to a movie theater, and uses them to wheedle free admittance and popcorn. He assures his children he misses them and hopes to return home soon. Mollie notices that Stuart and Rona are spending more time together. Overjoyed to see daughter Julie take her first steps, Mollie instinctively calls out to James before remembering he no longer lives there. As Mollie, Stuart, and Rona—all accountants—spend Saturday night doing their taxes, Mollie imagines James driving a sports car with a beautiful blonde. At the Baby Gym, Mikey and Eddie discuss their toilet training. When James Ubriacco stops by, he amuses the roomful of children by dancing like Elvis Presley in Jailhouse Rock. Later, as James drops the kids at home, Rona convinces him that Mollie wants to reconcile with him. Mikey yells for mommy and daddy to come see that he has finally gone to the toilet all by himself, and James and Molly are very proud of him. After James is called out on a night job, Mollie worries about the terrible weather. She telephones the airport, but James is already in his jet. She takes a taxi to the airport, runs to the airplane, and refuses to leave until James promises he will not fly in the storm. An argument is averted when the air control tower cancels all flights. James embraces Mollie, realizing she still loves him. Meanwhile, a robber breaks into their apartment. Stuart grabs his gun and chases the thief down a fire escape, but flames ignite on the kitchen stove. James and Mollie arrive home in a taxi, as Stuart fights the burglar in the middle of the street. James knocks the burglar unconscious, and police arrive to arrest the criminal. Seeing flames in their window, Mollie and James run into their apartment building. The elevator doors open and Mikey pushes Julie’s stroller into the lobby. James hurries upstairs to put out the fire with an extinguisher. Later, the whole family dines at Mollie’s parents’ house, and everyone seems happy. Mikey takes Julie aside, pledges his brotherly love, and tells her they have to stick together from now on.