In 1978, a young Dave Buznik (Adam Sandler) is on the brink of sharing a kiss with the girl of his dreams when a local bully, Arnie Shankman, embarrassingly yanks down his pants and underwear, leaving a lasting emotional scar that haunts Dave's ability to show affection publicly. Fast forward to present-day New York, where Dave works a mundane job as a secretary for his tyrannical boss Frank, who constantly steals credit for Dave's hard work. This tension spills into his personal life as well; his girlfriend Linda (Marisa Tomei) is worried about their relationship, especially with her ex-boyfriend Andrew hanging around and belittling Dave at every turn.
During a flight to a business meeting, Dave finds himself seated next to the notoriously annoying Buddy Rydell (Jack Nicholson). As Buddy's incessant chatter reaches a breaking point, Dave requests a headset to block him out, only to have the flight attendant mishandle the situation. In a bizarre turn of events, this leads to an overreaction from an airborne sky marshal who, thinking he's keeping the peace, tasers Dave. Consequently, Dave is arrested, tried, and sentenced to anger management therapy—a fate he never anticipated.
At his first therapy session, Dave is confronted with the shocking news that Buddy is in fact his therapist. As he is introduced to a varied group of individuals grappling with their own mental health issues, Dave's irritation with Buddy's unconventional methods grows. Following the session, Buddy informs Dave that he must double his therapy attendance to address his latent passive-aggressive tendencies. In a turbulent night out at a bar with his "Anger Ally" Chuck, played by John Turturro, a scuffle breaks out, and in an attempt to intervene, Dave accidentally hits a waitress, landing him back in court.
Things escalate when Buddy moves into Dave's apartment to oversee his therapy. Buddy's presence aggravates Dave in numerous ways, including sabotaging his professional life by insulting his boss and belittling Andrew, who is connected to Frank. When Dave jokingly exaggerates a call about Buddy's mother needing surgery, he quickly regrets it, confessing to Buddy, who slyly vows to teach Dave a lesson in revenge. The duo heads to Boston under the guise of visiting Buddy's mother, where they detour to a restaurant, and in a moment of weakness, Buddy pushes Dave to meet an attractive woman. However, Dave chooses loyalty to Linda over casual romance, only to later discover Buddy has spilled the beans about the encounter, further deepening his woes.
Their road trip takes a wild twist when Buddy leads Dave to confront Arnie, who has since turned into a monk. With Buddy's meddling lies fueling tensions, conflicts arise, eventually leading to a brawl where Dave emerges victorious against Arnie. Back in New York, Dave gathers the courage to propose to Linda but loses his nerve as their relationship faces strain. When he later discovers Buddy is dating Linda, anger bubbles over, resulting in an assault on Buddy that lands him back in court once more.
As fate twists again, Dave learns that Buddy is taking Linda to a Yankees game, leading him to assume Buddy is stealing his proposal idea. He rushes to the stadium and runs onto the field, where security apprehends him, but in a stroke of luck, Mayor Rudy Giuliani intervenes. In an impromptu moment, Dave proposes to Linda, who challenges him to prove his love with a kiss in front of thousands. Upon compliance, she acknowledges his growth and declares that he has successfully navigated his anger management trials.
Unbeknownst to Dave, the entire series of events had been orchestrated by Buddy, who meticulously planned this journey of self-discovery with the help of various participants, including the flight attendant and even the waitress. The film culminates in a joyous celebration in Central Park, where Buddy's past catches up with him when a man approaches with a gun. However, Dave steps up, recalling the lessons learned from Buddy. The anxious moment culminates in laughter when the "gun" turns out to be a water toy. The group harmonizes together, signifying the end of a tumultuous yet transformative journey.
