Donald Morton (Josh Hartnett) is a taxi driver and drives two Japanese passengers and his pet cockatiel around Spokane, Washington. Distracted, he bumps into the back of a florist’s van and damaging his stock. Unfazed, Donald and his budgie take their groceries and leave, abandoning his taxi cab and passengers. He takes his groceries to the self-help group for autistic adults. Before they head to the park to meet another autistic group, he tells one member, Gracie (Rusty Schwimmer), to gather the women and he’ll gather the guys to practice telling personal stories, but keeps getting distracted by performing mathematical sums of the microwaves depleting numbers. He notices that Isabelle Sorenson (Radha Mitchell), a new name, has signed up and tells Gracie to let her go first.
At the park, Isabelle tells of a childhood memory to the women: she saw that her parents were happy that an Olympian had broken a record, so in order to please her parents, and taking what she heard literally, she broke their music records. Donald tells his story to the men about his ability to do complex sums but couldn’t make friends. Isabelle goes on to tell of when she was raped when she went hitch-hiking. This causes Gracie to laugh manically. Heard by Donald, he tries to calm an angry Isabelle down and they find that they have much in common and take a liking to each other.
In the self-help group, after Isabelle talks to Gregory (John Carroll Lynch), a man who is writing notes on his notepad, about Donald, he calls Donald over to ask her to escort him to the Halloween party. Before Donald can, Isabelle asks Donald out for lunch. They go to the zoo the following day, and in response to Donald asking on behalf of Gregory, Isabelle asks Donald to escort her. They agree to meet with their costumes on in the evening. Donald dresses as a whale but is hesitant about going and leaves Isabelle, dressed as Mozart, waiting. She goes to his apartment and they both walk around the town talking until the final bus is due, and they share their first kiss.
Unsure when to call, Donald leaves multiple messages on her phone until she finally answers and they go to the amusement park. In the ring toss, the clanging of the metal rings hitting the bottles and the bell ringing cause her to scream and collapse on the floor. He takes her back to his filthy apartment and they agree to sleep together. The following day at the self-help group, Gregory accuses Donald of exploiting his position for sexual favors. Isabelle makes herself liked by going with Bronwin (Erica Leerhsen), who learns that her father has blood cancer and hired a guy for hitman absolution trainer to kill him, to wait for her parents to pick her up.
Isabelle takes the liberty of cleaning his apartment while he goes shopping. When he returns, he’s horrified to see that everything is different; the piles of newspaper are stacked neatly, rotting food from the fridge is thrown away and has a new shower curtain. He gets angry at Isabelle for changing everything. He later leaves a number of apologetic messages on Isabelle’s phone. The next day, he goes to the hair salon where Isabelle works as a hair stylist to apologize in person, and Isabelle forgives him, introducing him as her boyfriend to the staff.
Isabelle shows Donald an abandoned rooftop, calling this a place where people who don’t know where they belong can belong. She suggests that they can buy a house and her therapist has organised a job interview for a statistic analyst post at a university. He gets the job and they move into their new house, making it their own.
Donald tells Isabelle that he wants everything to be “nice” for when his boss, Wallace (Gary Cole) comes for dinner. Believing that he thinks that she doesn’t keep the house “nice,” Isabelle spites Donald by keeping the pets uncaged, much to Donald’s shock when he returns, and she maintains extroverted behavior and tells of her off-the-wall plans for the house. Donald explodes, but when Isabelle says that they are both crazy, he retaliates by telling her that she is crazier, which leads to her throwing him out. He stays with Gregory in his house, and after listening to an answer message that Isabelle’s rabbit, Bongo, has died, he runs to comfort her. Isabelle suggests that they should just be friends.
Donald invites Isabelle to a restaurant, where he proposes to her, much to Isabelle’s dismay. She leaves abruptly back home and overdoses. Donald returns just in time to take her to the hospital, where Isabelle’s psychiatrist advises him to leave her alone, testing his willpower to refrain from calling her.
Donald sees Isabelle leaving the university and follows her to the abandoned rooftop, where he expresses that the only nice thing he had left to give her was not to call, to find that Isabelle was waiting for his call and she missed him. They express their true love with an embrace and kiss. The movie ends with the happy couple in their home, enjoying Thanksgiving dinner with the self-help group.