Seymour Irving Levov is born and raised in the Weequahic section of Newark, New Jersey, in 1927 as the elder son of a successful Jewish American glove manufacturer, Lou Levov, and his wife Sylvia. Called "the Swede" because of his anomalous blond hair, blue eyes and Nordic good looks, Seymour is a star athlete in high school; a two-year veteran of the Marine Corps; and the narrator Nathan Zuckerman's idol and hero. Zuckerman and Seymour's younger brother, Jerry—who grows into a curmudgeonly, irascible heart surgeon with little empathy for the Swede—are schoolmates and close friends. The Swede eventually takes over his father's glove factory and marries Dawn Dwyer, a former beauty queen from nearby Elizabeth, whom he met in college.
Seymour establishes what he believes to be a perfect American life with a beloved wife and daughter, a satisfying business career, and a magnificent house in the idyllic hamlet of Old Rimrock. Yet, as the Vietnam War and racial unrest rack the country and destroy inner-city Newark, his precocious teenage daughter Meredith ("Merry"), beset by an emotionally debilitating stutter and outraged by the war, becomes grotesquely overweight and increasingly radical in her beliefs. In February 1968, Merry plants a bomb in the Old Rimrock post office, which kills a bystander; she goes into permanent hiding. Seymour finds Merry five years later, living in deplorable conditions in inner-city Newark. During this reunion, Merry reveals that she was responsible for several more bombings, killing three more people. Though Merry informs him that her actions were deliberate, Seymour decides to keep their meeting a secret, believing Merry has been manipulated by an unknown political group and a mysterious woman named Rita Cohen.
At a dinner party, Seymour discovers that his wife Dawn has been having an affair with Princeton-educated architect William Orcutt III, for whom she undergoes a facelift. Seymour then realizes that his wife is planning to leave him for Orcutt. It is revealed that Seymour himself previously had a short-term affair with Merry's speech therapist, Sheila Salzman, and that she and her husband Shelly hid Merry in their home after the post office bombing. Seymour sadly concludes that everyone he knows may have a veneer of respectability, but each engages in subversive behavior and that he cannot understand the truth about anyone based upon the conduct they outwardly display. He is forced to see the truth about the chaos and discord rumbling beneath the "American pastoral," which has brought about profound personal and societal changes he no longer can ignore. Simultaneously, the dinner party underscores the fact that no one ever truly understands the hearts of other people.