Sam Dawson (Sean Penn), a man with an intellectual disability, is the single father of Lucy (Dakota Fanning), following their abandonment by her mother, who is revealed to be a homeless woman. Sam is well-adjusted and has a supportive group of friends with disabilities, as well as a kind, agoraphobia neighbor Annie (Dianne Wiest) who takes care of Lucy when Sam cannot. Though Sam provides a loving place for precocious Lucy, she soon surpasses his mental ability.