New York taxi driver Donald Morton (Josh Hartnett) has Asperger's syndrome, a type of autism. Proficient with numbers but shy around people, Donald organizes a support group for others with with his condition. Isabelle Sorenson (Radha Mitchell), who is direct to a fault and practical, but easily upset by any perceived insult, joins the group. The two fall in love, but their mutual difficulty in coping with their condition threatens their fragile relationship.