At risk of having to owe a significant amount of money he doesn't have due to his imported cars not passing the emission standards set by the Environmental Protection Agency, small-time car dealer Charlie Babbitt (TOM CRUISE) does not have time to worry about anything beyond how to fix the problems looming in front of him that threaten to put him out of business. His girlfriend Susanna (VALERIA GOLINO), who works with him at this business, notes his silence and asks him to open up to her, but he won't. He never has. In fact, it looks like he's never opened up to anyone in his life.
At the worst possible moment, when he and Susanna are on the road to a much-needed-or-insanity-reigns weekend in Palm Springs, he receives a call that his father died, whom he has been estranged from for years. With Susanna, he goes back to Ohio to learn that his father left him his Buick and his prized rose bushes, but not what Charlie sees as the big prize, the one that could help him pull his sinking business up from rising waters: $3 million dollars.
That money, he learns, goes to an unnamed beneficiary, which, after some brief strategic detective work, leads him to Wallbrook, an institution for the severely disabled. It's there that Charlie learns that he has a brother, Raymond (DUSTIN HOFFMAN), an autistic savant. Charlie is not happy at this development, particularly since he never knew about Raymond, even in the years he was growing up.
But he has a plan: Take Raymond with him from the institution and refuse to bring him back unless he gets half of the three million dollars, which causes Susanna to leave him for a time until he can learn to open up and be a better person. Getting back to Los Angeles right away to attend to his failing business is impossible because of Raymond's vehement aversion to flying, so Charlie is forced to make this a road trip and is unhappy about what he sees as a huge burden.