Sam Dawson (SEAN PENN) is a mentally challenged man who works cleaning tables at the local Starbucks. He's also the single father to Lucy (DAKOTA FANNING), his precocious 7-year-old who has accepted that he's different from other dads.
With the limited help of Annie (DIANNE WIEST), his shut-in neighbor, and Ifty (DOUG HUTCHISON),
Robert (STANLEY DESANTIS), Brad (BRAD ALLAN SILVERMAN) and Joe (JOSEPH ROSENBERG), his mentally challenged best friends, Sam has managed to raise Lucy to be an intelligent and well-adjusted young girl.
Yet, when she finally surpasses him intellectually, she begins to hold back in school, a point that eventually brings social worker Margaret Calgrove (LORETTA DEVINE) into the picture. She and others determine that Sam will be unable to raise Lucy properly in the coming years and thus remove her from his custody.
Desperate and prodded on by his friends to hire someone to defend his rights, Sam approaches Rita Harrison (MICHELLE PFEIFFER), a harried lawyer who knows he can't afford her and that she neither has time for him nor his case. Peer pressure, however, eventually makes her change her mind, and she reluctantly sets out to help him.
As she gets to know Sam and Lucy goes off to live with her foster mom, Randy (LAURA DERN), Rita then tries to do what she can - while taking on Mr. Turner (RICHARD SCHIFF), her opposition in the custody hearings - to prove that Sam's a good parent and is capable of continuing to raise Lucy.