Jason "Igby" Slocumb Jr. is a disenchanted 17-year-old boy striving to break free from the stifling environment imposed by his wealthy East Coast family. He harbors a deep-seated fear of becoming mentally unstable like his father, Jason, who has been institutionalized due to schizophrenia. His mother, Mimi, is notably self-absorbed and emotionally distant, often turning to alcohol to cope. Igby derisively labels his ambitious older brother Ollie a fascist, bitterly describing his studies in "neo-fascism" (economics) at Columbia University.
Determined to discover a more fulfilling existence, Igby rebels against his family at every turn. After conveniently flunking his way through several private schools, he finds himself at a harsh military academy, where he endures bullying from fellow cadets. Following his escape, he spends time at a Chicago hotel funded by his mother's credit card, only to be sent to New York for the summer to stay with his godfather, D. H. Banes, a prosperous real estate magnate.
During his construction job for D. H., Igby meets Rachel, an Edie Sedgwick-like trophy mistress who is grappling with heroin addiction. Rather than return to the academic pressures of school, he immerses himself in the vibrant yet tumultuous bohemian scene of Manhattan, finding solace with Rachel and her performance artist friend Russel at her D. H.-owned studio. Their relationship escalates to a sexual encounter, but when D. H. discovers them, Rachel reacts violently, concerned for her precarious living situation. Following this, Igby briefly reconnects with Sookie, a part-time lover, who eventually leaves him for Ollie, intensifying his sense of betrayal.
Although Mimi appears aloof, she is not entirely unmoved by Igby's choices. She recalls his conception as an act of resentment, suggesting that it’s hardly surprising that his life has taken such a troubled path. His name, a family in-joke, derives from his childhood mispronunciation of his toy bear, Digby, to whom he foolishly assigned blame for his misdeeds. The family playfully dubbed him "Igby" whenever he failed to take responsibility.
Igby's world is upended when D. H. reveals that Mimi is gravely ill with breast cancer, prompting him to return home to confront her. She requests his presence during her final moments, having arranged her own death with Ollie's assistance, who administers drug-laden yogurt before ultimately suffocating her with a plastic bag.
Before her passing, she casually discloses a shocking truth to Igby: "I take it you know that D. H. is your father?" After this revelation, Igby visits his father, Jason, in the hospital, gearing up to leave for Los Angeles, determined to sever ties and embark on a fresh start, 3,000 miles away from his tumultuous family legacy.
