Charlie Babbitt, a self-centered and abrasive car dealer in Los Angeles, learns that his estranged father has died. Expecting a substantial inheritance, Charlie is shocked to discover that the estate, worth millions, has been left to an unknown trustee. In pursuit of his inheritance, Charlie uncovers that the money is being directed to a mental institution, which is home to his older brother Raymond, an autistic savant whom Charlie never knew existed. Raymond has exceptional memory and mathematical abilities but struggles with understanding emotions and social cues. Hoping to gain legal custody and access to the estate, Charlie impulsively decides to take Raymond out of the institution.

The brothers embark on a cross-country road trip that becomes a journey of self-discovery. As they travel, Charlie's frustration with Raymond's peculiarities leads to tension between them. However, during their time together, Charlie begins to see past Raymond's disabilities and starts to understand his brother's unique view of the world. The trip becomes a transformative experience for Charlie as he develops a genuine connection with Raymond and confronts his own shortcomings and past grievances. Through highs and lows on the road, including the challenges of adapting to one another's needs, the brothers' relationship slowly evolves, shedding light on the true meaning of family and empathy.