Justin Cobb (Lou Pucci) still sucks his thumb at 17. He wants to stop and knows that it's disrupting his family, his love life and his identity. But thumbsucking is only a symptom of a deep-seated fear: that he and his father (Vincent D'Onofrio) aren't good enough for his mother (Tilda Swinton), and that she'll leave them. Justin spirals into manic behavior, is diagnosed with ADHD and put on Ritalin, which becomes a substitute for his thumb.
His mother wonders how she could possibly be "grown up" with a son going to college. In her early 40s, she is, like Justin, struggling to find out who she is and to accept her shortcomings. Audrey hides from these realities through an obsession with a TV heartthrob, Matt Schraam (Benjamin Bratt), whose TV character seems to have all the neat answers.
Little brother Joel (Chase Offerle) is mostly oblivious to the family's dynamics, and thinks his older brother is embarrassing. Justin turns to others outside his family to help. His New Age orthodontist, Perry Lyman (Keanu Reeves), seems to mirror Justin's ups and downs. They both try on different personas that hide their doubts and fears.
At school, Mr. Geary (Vince Vaughn), Justin's debate coach, initially tries to help. Striving to act as a peer to Justin, Mr. Geary eventually finds that he can no longer control him when Justin reaps great success.