Justin Cobb is a shy 17-year-old in a family of four in suburban Oregon. He has a persistent thumb-sucking habit his father disapproves of, which has led to major orthodontic repair. He addresses his parents by their first names, Mike and Audrey, so as not to make his father feel old. Audrey, a registered nurse, is idly fascinated by actor Matt Schramm, entering a contest to win a date with him. She insists it is "innocent fun", but is inordinately concerned with looking attractive for the contest.
Justin struggles on his school's debate team, led by Mr. Geary which he joined to get closer to his environmentalist classmate Rebecca. He tries to start a relationship with her, but she rejects him after he cannot open up to her about his thumb-sucking habit.
At a regular checkup, Justin's orthodontist, Dr. Perry Lyman, indicates he can tell that Justin is still sucking his thumb, and attempts hypnosis, coaching Justin to find his power animal (a deer) and suggesting that his thumb will taste like echinacea. This works, and Justin finds his thumb distasteful, but falls deeper into frustration without the crutch. After Justin conspires with his brother to disrupt Dr. Lyman in a bicycle race with Justin's father, his school counselor prods the Cobbs to give him Ritalin. While his parents wring their hands over the idea, Justin insists that he needs the help.
Almost immediately after beginning treatment, Justin begins to have elevated energy, confidence and focus. He begins to excel on the debate team, unseating Rebecca from the star position; she quits the team and drifts into the stoners crowd. Justin's newfound aggressiveness nets the debate team repeated awards. Simultaneously, he begins to challenge the neuroses of the adults around him, especially their struggles with aging. With a somewhat deceitful cover letter, he applies to NYU, in spite of his mother's urging that he go to college closer to home.
After rambling incoherently at the state debate championship, Justin quits the debate team, throws away the pills, and seeks out Rebecca to hook him up with pot. During their smoking sessions, Rebecca blindfolds him and engages with him in kissing and other sexual activity, which Justin interprets as a relationship. But when he broaches the subject, Rebecca tells him otherwise, calling their meetings an "experiment." He quits both her and the drugs.
Both Justin and his father suspect that Audrey is having an affair with Schramm after she is transferred to a celebrity rehab facility where Schramm has been committed. Attempting to catch his mother in the act, he instead meets Schramm sneaking a smoke in the bushes, and learns the unromantic truth. The next day, he receives an acceptance letter from NYU.
During a final checkup, Dr. Lyman reveals to Justin his discovery that thumb-sucking is not a medically debilitating problem, and says that everyone has their own flaws and nobody has all the answers—that in fact learning to live without having the answers is (perhaps) the answer. On his flight to New York, Justin dreams of reaching his goal of being a TV anchor, "sharing the truth with the world". He wakes up after sleeptalking to find his thumb in his mouth and an attractive girl smiling at him. Slightly embarrassed but self-confident, he introduces himself.Lou Taylor Pucci as Justin Cobb
Colton Tanner as 10-year-old Justin
Tilda Swinton as Audrey Cobb
Vincent D'Onofrio as Mike Cobb
Keanu Reeves as Dr. Perry Lyman
Kelli Garner as Rebecca
Benjamin Bratt as Matt Schramm
Vince Vaughn as Mr. Geary
Chase Offerle as Joel Cobb
Kit Koenig as Principal
Nancy O'Dell as herself
Walter Kirn (cameo) as Debate judge
Review aggregator Rotten Tomatoes gives Thumbsucker a score of 71% based on 108 reviews, with an average rating of 6.7/10. The site's consensus is: "Though quirky coming-of-age themes are common in indie films, this one boasts a smart script and a great cast."
Steven Rea of the Philadelphia Inquirer gave it three-and-a-half stars out of four, calling it a "quiet, quirky gem" and "terrific".
Roger Ebert of the Chicago Sun-Times gave it three stars out of four, writing "I have focused on Justin, but really the movie is equally about the adult characters, who all seem to have lacked adequate parenting themselves. We talk about the tragedy of children giving birth to children; maybe that can happen at any age."
The film is set in the fictional town of Beaverwood, Oregon, and filming locations included Beaverton and Sherwood, Oregon, Portland International Airport, Tualatin High School, and the Living Enrichment Center in Wilsonville, Oregon.
The soundtrack to the film was originally to consist of a number of cover songs performed by Elliott Smith, but he died before the project's completion. Tim DeLaughter and The Polyphonic Spree were then chosen to compose an original soundtrack after Mills attended one of their shows and was impressed. Three of Smith's songs remain on the soundtrack.