In the sleepy Endora, Iowa, Gilbert Grape and his younger, mentally handicapped brother Arnie watch a caravan of Airstream trailers passing through town. Gilbert muses that Arnie was not supposed to live past the age of ten, but the mischievous teenager is about to turn eighteen. Gilbert and Arnie live with older sister, Amy, fifteen-year-old Ellen, and their morbidly obese mother, “Momma,” in a ramshackle house built by their dead father. An older brother named Larry is estranged from the family. With Amy, Gilbert bears the burden of taking care of Arnie, who has a penchant for running away, and Momma, who is confined to the living room couch. Gilbert languishes in his job at Lamson’s Grocery, a struggling “mom-and-pop” store. One day, he delivers groceries to Betty Carver, with whom he is having an affair. He leaves Arnie in the car, and Betty sends her sons outside to play. Betty and Gilbert kiss, but stop abruptly when Mr. Carver arrives home. Gilbert flees, and finds Arnie has run off to climb the local water tower – a prank he regularly pulls. Gilbert arrives at the tower, and uses a police megaphone to coax his brother down. Sheriff Jerry Farrel reprimands Gilbert, warning that Arnie cannot do this again. At dinner, as Amy and Ellen discuss plans for Arnie’s eighteenth birthday party, Ellen accuses Gilbert of not contributing. Gilbert works on house repairs with his friend, Tucker Van Dyke, who hopes to secure a job at Burger Barn, a new fast food restaurant coming to town. When a young boy stops by the house to gawk at Momma, Gilbert helps him sneak a peek at the obese woman, but Tucker disapproves. At work one day, Gilbert encounters an out-of-towner named Becky, who was traveling with her grandmother in the Airstream caravan, but is now stuck in Endora, waiting for a spare part to fix her grandmother’s truck. Becky buys groceries, and Gilbert and Arnie give her a ride home. Although she is patient and kind to Arnie, Gilbert is too nervous to speak to Becky. That night, Arnie upsets Momma when he chants “Dad is dead” at the dinner table. Momma stomps on the floor, shouting for him to stop, and Gilbert notices the floorboards buckling under her weight. He enlists the help of Tucker Van Dyke, who erects several support beams in the basement. Tucker asks for help, and Gilbert tries to send Arnie down, but he refuses. Tucker recalls that, seventeen years ago, their father hanged himself in the basement, and apologizes to Gilbert for not remembering. Frustrated with his chaotic family life, Gilbert visits Becky and her grandmother. Betty Carver grows jealous when she spots Gilbert and Becky together at an ice cream shop. Later, they retreat to a field to watch the sun set. Gilbert runs home to bathe Arnie. In his haste to get back to Becky, he leaves the boy to get out of the bath and dry himself. The next morning, Gilbert awakens to find Arnie still in the bathtub, shivering. Momma embraces the traumatized Arnie and chastises Gilbert for his negligence. To celebrate her and Gilbert’s “anniversary,” Betty Carver calls for a grocery delivery. Gilbert arrives at her home, and she asks him to dial a number because her hands are covered in cookie dough. Gilbert dials and is shocked to hear Betty’s husband on the line. As Betty initiates oral sex, Mr. Carver asks Gilbert to come to his office. There, instead of accusing Gilbert of sleeping with his wife, he tries to sell him insurance. Betty calls in a panic and demands that her husband come home. Carver nervously admits to having family troubles. That night, Carver is found dead in the “kiddie” pool in his front yard. Gilbert’s friend, an undertaker named Bobby McBurney, conjectures that the man had a heart attack and fell into the pool, rather than drowning as police reported. Gilbert spends more time with Becky. She asks what he wants in life, and he answers that he wants a new house for his family, a new brain for Arnie, and for his mother to take aerobics classes. Their conversation is interrupted when Gilbert realizes Arnie has run away again. He finds him at the water tower. This time, Arnie climbs all the way to the top and police must use a crane to rescue him. Sheriff Jerry Farrel arrests the boy. For the first time in years, Momma leaves the house to go to the police station. There, she yells at Sheriff Farrel and refuses to fill out paperwork, demanding that Arnie be released immediately. Farrel complies. On their way out of the station, Momma and her children encounter a crowd of voyeurs, who gawk and snap photographs of the large woman. Becky tells Gilbert she would like to meet his mother, but he rejects the idea. During Mr. Carver’s funeral, Arnie sees the prefabricated Burger Barn being shipped into town and interrupts the solemn event with excited screams. The widowed Betty Carver tells Gilbert she is moving to St. Louis, Missouri, and bids him goodbye. At the Burger Barn grand opening, Tucker Van Dyke proudly dons a uniform and serves burgers to his friends. Becky finds Gilbert and tells him her grandmother’s truck is fixed, and they are leaving the next day. Gilbert reacts coldly. At home, he tries to get Arnie to take a bath, but Arnie refuses. Gilbert loses his temper and strikes the boy several times, drawing blood. He flees the house and drives out of town. Later that night, he goes to see Becky and finds her swimming in a pond with Arnie. After Amy and Ellen retrieve Arnie, Becky and Gilbert spend the night together. Gilbert finally opens up to her about his father’s suicide. The next day, at Arnie’s birthday party, Gilbert finds his brother and apologizes. Momma, who is hiding out in a bedroom, is less quick to forgive him. She breaks down in tears, and tells Gilbert she never meant to become like this. Gilbert assures her that he is not ashamed of her. Becky arrives, and Gilbert convinces Momma to meet her. Becky shakes Momma’s hand and sets her at ease. Back outside, she bids Gilbert goodbye and they embrace. After the party, Momma decides to climb the stairs to her old bedroom. Finally making it to her old bed, she collapses and calls for Arnie. The boy runs upstairs, and finds her dead. Certain that police will need a crane to remove Momma’s body, Gilbert decides to preserve her dignity by burning the house down. He and his siblings move the furniture outside and set the house ablaze. A year later, Amy takes a job managing a bakery in Des Moines and arranges to move there with Ellen. When the yearly Airstream caravan passes through town, Arnie and Gilbert join Becky and her grandmother, and ride out of town.