The novel is about the life of T. S. Garp. His mother, Jenny Fields, is a strong-willed nurse who wants a child but not a husband. She encounters a dying ball turret gunner known only as Technical Sergeant Garp, who was severely brain damaged in combat. Jenny nurses Garp, observing his infantile state and almost perpetual erection. Unconstrained by convention and driven by her desire for a child, Jenny impregnates herself by raping the brain-damaged Garp once, and names the resulting son "T. S." (a name derived from "Technical Sergeant", but consisting of just initials). Jenny raises young Garp alone, taking a position as the live-in nurse at the all-boys Steering School in New England.
Garp grows up, becoming interested in sex, wrestling, and writing fiction—three topics in which his mother has little interest. After his graduation in 1961, his mother takes him to Vienna, where he writes his first novella. At the same time, his mother begins writing her autobiography, A Sexual Suspect. After Jenny and Garp return to Steering, Garp marries Helen, the wrestling coach's daughter, and begins his family—he a struggling writer, she a teacher of English. The publication of A Sexual Suspect makes his mother famous. She becomes a feminist icon, because feminists view her book as a manifesto of a woman who does not care to bind herself to a man, and who chooses to raise a child on her own. She nurtures and supports women traumatized by men, among them the Ellen Jamesians, a group of women named after an eleven-year-old girl whose tongue was cut off by her rapists to silence her. The members of the group cut off their own tongues in solidarity with the girl (the girl herself opposes this tongue cutting).
Garp becomes a devoted parent, wrestling with anxiety for the safety of his children. He and his family inevitably experience dark and violent events through which the characters change and grow. Garp learns (often painfully) from the women in his life (including transgender ex-football player Roberta Muldoon), who are struggling to become more tolerant in the face of intolerance. The story contains a great deal of (in the words of Garp's fictional teacher) "lunacy and sorrow".
The novel contains several narratives: Garp's first piece of fiction, a short story entitled The Pension Grillparzer; Vigilance, an essay; and the first chapter of his third novel, The World According to Bensenhaver. The book also contains some motifs that appear in other Irving novels: bears, New England, Vienna, hotels, wrestling, a person who prefers abstinence over sex, and adultery.
