It's not easy growing up, no matter what age you are...
Justin Cobb (Lou Pucci) still sucks his thumb at 17. He wants to stop and knows that his thumbsucking is disrupting his family, his love life and his identity. The only thing that changes his behavior is hypnosis therapy administered by his "guru" orthodontist. But while Justin felt this would solve all his problems and he would finally be "normal," his troubles were really just beginning. Thumbsucking was only a symptom of a deep-seated fear: that he and his father aren't good enough for his mother, and that she would leave them. Thumbsucking was the only way to soothe this fear. Once that is gone Justin spirals into manic behavior, is diagnosed with ADHD and put on Ritalin, which becomes a substitute for his thumb. When the high of the prescription drugs and a newfound success at school crash, Justin turns to pot and sex to replace his thumb.
His father Mike (Vincent D'Onofrio) is himself shadowed by the sad twists of youth: a broken college football career apparently sidelined by a knee injury. In many ways Mike feels like a teenager, yet he finds himself in his early 40s with his eldest son still sucking his thumb as he's heading to college and his wife seeming to drift away. Mike hides his fears and disappointments with tough behavior - but that trick is running dry. He is growing out of the lies and denials that got him from college to the present. Did he ever really want to be a jock? Is that a false self that he has hidden in for all these years and doesn’t know how to evolve out of?
His wife Audrey (Tilda Swinton) wonders how she could possibly be "grown up" with a son going to college. She seems to be asking "How did I get here?" and "Is this all there is?" In her early 40s, she is, like Justin, struggling to find out who she is and to accept her shortcomings. As a mother she knows that she doesn't have all the answers to Justin's troubles, and that she is sometimes too busy dealing with her own doubts to help him. 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, but his real drug addicted self reveals messier truths to both her and Justin.
Little brother Joel (Chase Offerle) is mostly oblivious to the family's dynamics, and thinks his troubled older brother is embarrassing. Joel provides comic relief with his lack of doubt in a family full of questions, yet his strong-mindedness may be misunderstood.
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. Perry transforms into an EST/FORUM-style high achiever before winding up a washed-up mess who stops looking for disguises and easy answers, finding a strange kind of happiness in the process.
At school, Mr. Geary (Vince Vaughn), Justin's debate coach, initially tries to help. His alternately dominating and cripplingly insecure behavior is revealed as he attempts to live his life through his students. Striving to act as a peer to Justin, Mr. Geary eventually finds that he can no longer control him when Justin reaps great success, and their relationship crumbles. Justin's heart belongs to another pretender, Rebecca (Kelli Garner) who masks her insecurities about sex and relationships behind a cool demeanor. She seems adult beyond her years, but Justin learns the hard way that she is not able to face her fears.
"Thumbsucker" is an honest and funny look at the struggles of people who feel deeply flawed, both those in youth and middle age. They yearn to be anything but their real selves with their real fears and doubts. Instead, they want to be "normal." They look for magic answers to fix themselves – only to realize that they can't be someone else, that "normal" does not exist, and that their flaws are what make them human and loveable.