Struggling to keep his specialty car dealership in business, Charlie Babbitt takes his fiancée and employee, Susanna, for a weekend getaway in Palm Springs, California. However, on the way there, Charlie learns that his father, Sanford Babbitt, has died, and he and Susanna travel to Cincinnati, Ohio, for the funeral. At his childhood home, Charlie shows Susanna his father’s 1949 Buick Roadmaster convertible that he coveted as a child. He recalls a traumatic incident when, as a teenager, he took the car against his father’s wishes. His father reported the vehicle stolen and allowed Charlie to stay in jail for two days after he was arrested. Upon his release, Charlie left town and never spoke to his father again. Later that day, a lawyer reads Charlie a bitter letter from his father, who left him the Buick Roadmaster and some rose bushes, while the rest of his $3 million estate has gone into a trust for another beneficiary. The next day, Charlie goes to his father’s bank and learns that the beneficiary resides at Wallbrook, an institute for the mentally disabled. There, Charlie meets with Dr. Bruner, who refuses to disclose any information regarding the trust. Meanwhile, an autistic man named Raymond approaches Susanna as she waits outside in the Buick Roadmaster, informing her that he drove the car just last week. When Charlie returns, he orders Raymond to go away but stops short when Raymond describes the history of the car in detail. Realizing that he and Raymond share the same parents, Charlie demands an explanation from Dr. Bruner, who confirms that Raymond is Charlie’s brother and has lived at Wallbrook since he was eighteen years old, and Charlie was still a toddler. He says Raymond is a high-functioning autistic savant, who has trouble communicating and expressing his emotions in a traditional way. To avoid feeling terrified by life, Raymond has established routines and rituals that include daily viewing of the television programs, Jeopardy! and The People’s Court. Due to Raymond’s autism, and his inability to understand the concept of money, Bruner explains that he has control over the $3 million trust – a fact that Charlie finds maddening. In his room, Raymond nervously recites the Abbott and Costello comedy routine, “Who’s on First?” while Charlie touches his books against his brother’s will. A caretaker named Vern explains that Raymond does not like his books to be touched, nor does he like to make physical contact with others. Although Jeopardy! will soon air, Charlie convinces Raymond to join him for a walk. Outside, he lets him know that their father has died and asks if he would like to visit the cemetery. Seeming confused, Raymond responds, “I don’t know.” Charlie tries to lure Raymond on a road trip by promising to take him to a Los Angeles Dodgers baseball game. Raymond agrees, but also says he is not supposed to be off the grounds for more than two hours. They leave Wallbrook and, later, check into a hotel. Raymond complains that his room looks different and demands to eat the same dinner he would have been served at the facility. Susanna argues with Charlie, who led her to believe that the trip was Dr. Bruner’s idea. Later that night, Raymond hears Susanna and Charlie having sex and wanders into their adjoining room. Charlie yells when he realizes his brother is sitting on the bed, then leads him back to his room, where Raymond uses a flashlight to read the phonebook in the dark. Susanna reprimands Charlie for being insensitive. Charlie then confesses that he plans to take Raymond back to his home in Los Angeles, and keep him until Bruner gives Charlie half his father’s estate. Disapproving, Susanna packs her things and leaves. The next day, Charlie and Raymond stop at a diner, where Raymond recognizes a waitress’s name from the phonebook and recites her phone number. Charlie realizes that Raymond memorized A-G in the phonebook overnight, then observes as his brother accurately counts 246 fallen toothpicks in a matter of seconds. That afternoon, as they are about to board a plane to Los Angeles, Raymond cites airplane crash statistics and refuses to board. When Charlie tries to force him, Raymond screams and beats his head until Charlie relents. They hit the road again in the Roadmaster, and the long trip to California is further slowed by Raymond’s quirks, including his refusal to go outside in the rain. Stopping in a small town one day, Charlie leaves his brother in the car to make a call and returns to find him gone. Meanwhile, Raymond has stopped in the middle of an intersection, misinterpreting a “Don’t Walk” sign, while cars honk at him. A man gets out of his car to push Raymond out of the way, but Charlie rushes to his aid. He takes Raymond to a doctor, who tests his intelligence by asking him to multiply large numbers and confirming the answers with a calculator. Based on his correct calcuations, Charlie determines that Raymond is a genius; however, when Raymond is asked to subtract simple dollar amounts, he is unable to apply math to real life situations. Over the next couple days, Charlie attempts to salvage his failing business over the phone and sets up a custody hearing for Raymond. The next time they check into a motel, Raymond mutters “Funny Rainman” and Charlie realizes that his childhood imaginary friend, “Rainman,” was actually his brother. Raymond produces a photograph of the two boys as children and recalls bidding Charlie goodbye as he left for Wallbrook. As Charlie draws a bath, Raymond beats his head and shouts, “Hot water burn baby,” prompting Charlie’s realization that his brother was sent away for nearly burning him in scalding water. The next day, Charlie buys Raymond a portable television and calls one of his salesman, Lenny, who informs him that his cars have been repossessed and the business owes $80,000 to customers. Desperate for money, Charlie trains Raymond to play blackjack and count cards, and takes him to a Las Vegas, Nevada, casino, where they quickly win $86,000. Charlie buys Raymond a date with a call girl and, at their lavish suite, teaches him how to dance in preparation for the date. Susanna shows up unexpectedly and reunites with Charlie. The three go down to the casino to meet Raymond’s call girl, but she does not show up. Taking Raymond back to the suite, Susanna dances with him in the elevator and teaches him how to kiss. The next day, Charlie lets Raymond drive outside the casino before they head to Los Angeles, where Raymond recites “Who’s on First?” as Charlie shows him around his home. Dr. Bruner arrives for the custody hearing, and offers Charlie a check for $250,000 to return Raymond back to his custody. However, Charlie refuses the check, claiming his priorities have changed and he wants to take care of his brother himself. As Charlie sleeps one morning, Raymond leaves a toaster on until smoke from the appliance sets off an alarm. Charlie wakes up to the sound of Raymond’s panic attack and rushes to stop the alarm. Later, as Raymond is interviewed by a psychologist for the custody hearing, he reveals that Charlie brought him to Las Vegas to gamble and allowed him to drive. When the psychologist asks Raymond where he wants to live, he simultaneously opts to stay with Charlie and return to Wallbrook. Realizing his brother needs more care than he can provide, Charlie allows Raymond to go back with Dr. Bruner, but promises to visit in two weeks. As they say goodbye at the train station, Raymond mentions the secret code he and Charlie used for betting on blackjack, then disappears onto the train.