Everything starts with Jenny Fields. Jenny was a feminist before feminists were cool—she's a working nurse with zero interest in men. She does, however, have interest in becoming a mother.
Enter the poor Technical Sargent Garp, a wounded soldier who ends up in Jenny's care. Garp, despite his child-like mental state, is still quite interested in sex. And so the proudly asexual Jenny Fields makes a baby with her ecstatic patient just weeks before his death. She names him T.S. Garp.
Jenny gets a job as a nurse at the Steerling School, an all-boys academy that Garp eventually attends. He joins the wrestling team and falls in love with Helen, his coach's daughter, and since Helen is an avid reader, Garp decides to become a writer at a young age. He ends up losing his virginity to Cushie Percy, however, and then he doesn't even see Helen for some time.
See, Garp and Jenny are going to Vienna so Garp can fulfill his dream of becoming a writer. Jenny secretly wants to become a writer, too, though, and during their trip, she ends up finishing her autobiography, A Sexual Suspect, which catapults her to fame on their return to America. Garp finishes his first work, too—a short story called "The Pension Grillparzer"—and although it doesn't make him as famous as Jenny, it's good enough to convince Helen to marry Garp. Not bad for a first try.
These next years are full of changes. Garp and Helen have their first son, Duncan, and Garp writes his first novel, Procrastination. Although their marriage looks solid on the surface, Garp cheats on Helen multiple times, culminating in a bizarre partner-swapping ordeal. Feelings are hurt and hearts are broken, but at least Garp has the inspiration behind his next novel—The Second Wind of the Cuckold.
Things only get worse from there, though, and Garp stops writing altogether. The one bright spot is the birth of their second son, Walt. The memory of Garp's affairs takes its toll, however, when Helen has her own affair with a grad student. Garp finds out and demands that Helen end their relationship. This leads to a car accident that hurts Helen and Garp, causes Duncan to lose an eye, and kills Walt. Ugh.
The road to recovery is a long one. The family stays at Dog's Head Harbor, Jenny's hospital and home for wayward women. The couple manages to rebuild their relationship and have a daughter named, um, Jenny. Garp finally starts writing again, channeling his trauma into a heart-breaking novel called The World According to Bensenhaver, which launches him to literary success and controversial infamy.
Then, just as things are looking up, tragedy strikes again. First, Jenny is assassinated while campaigning for a feminist-leaning gubernatorial candidate. Then, several years later, Garp is shot and killed by Pooh Percy, the nutty younger sister of Cushie who's convinced that Garp killed her. Despite these tragic ends, though, the book makes it clear that both Garp and Jenny's legacies will live on into the future. Go team.