Seymour Irving Levov is born and raised in the Weequahic section of Newark, New Jersey, in 1927 as the son of a successful Jewish American glove manufacturer, Lou Levov. Called "the Swede" because of his anomalous blond hair, blue eyes and Nordic good looks, Seymour is a star athlete in three sports at Weequahic High School, a Marine Corps veteran of World War II, although he actually does not get to participate in the war, 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, Newark Maid, and marries Dawn Dwyer, an Irish Catholic winner of Miss New Jersey from nearby Elizabeth, whom he met at Upsala College in East Orange, New Jersey.
Seymour establishes what he believes to be a perfect American life with a beloved family, a satisfying business career, and a beautiful old home in rural Old Rimrock, New Jersey. Yet, as the Vietnam War and racial unrest wrack the country and destroy inner-city Newark, Seymour's precocious teenage daughter Meredith ("Merry"), beset by an emotionally debilitating stutter and outraged at the United States' conduct in Vietnam, becomes more radical in her beliefs. In February 1968, at age 16, Merry commits an act of political terrorism. In protest against the Vietnam War and the "system," she plants a bomb in the Old Rimrock post office and the resulting explosion kills a bystander. In this singular act, Seymour is cast out of the seemingly perfect life he has built and thrown instead into a world of chaos and dysfunction. Like a number of real-life members of the Weather Underground, Seymour's daughter goes permanently into hiding. In Zuckerman's narration, a reunion of father and daughter takes place five years later, in 1973, in the squalor of Newark's ruined inner city, where a disheveled Merry, now age 21, has been living under the most deplorable conditions. During this reunion, Merry reveals that since the first bombing she had set off several other bombs, resulting in three more deaths, and that she has been repeatedly raped while in hiding. Though Merry informs him that she acted consciously and willingly in all four murders, Seymour decides to keep their meeting a secret, refusing to give up the notion of his daughter as basically an innocent young woman who has been manipulated and radicalized by stronger influences in the form of an unknown political group and a mysterious woman named Rita Cohen.
Zuckerman concludes his narration at a 1973 dinner party in the Levovs' home with Seymour's parents and several friends, during which Seymour discovers that his wife Dawn has been having an affair with a mutual friend and party attendee—a Princeton-educated blueblood, William Orcutt III, for whom she undergoes a facelift. Seymour then realizes that his wife is planning to divorce him and marry Orcutt (whose alcoholic wife will be dumped by him as well), and that Seymour's many efforts to help his wife surmount the tragedy of the daughter's behavior, including building her a dream house, have come to naught. The narrator also reveals that Seymour himself had had a four-month-long affair with Merry's speech therapist, Sheila Salzman, wife of a family practitioner, Dr. Shelly Salzman. The Salzmans, who are also at the party, had been responsible for hiding Merry in their home for three days after the first bombing five years earlier. During the party, the normally kind-hearted, unflappable, mild-mannered Seymour angrily confronts Sheila in the study, demanding to know why the Salzmans refused to turn Merry in instead of harboring her. Seymour sadly concludes that while all those at the party may have a veneer of respectability, each participates in his or her own way in subversive behavior (which Zuckerman refers to as the "substratum of the mind"), and that he is unable to understand the truth about anyone based upon the conduct they outwardly display. In this final scene, Zuckerman reveals that Merry's actions have at long last forced the stoic Seymour to see the truth about the chaos and discord rumbling beneath the "American pastoral," chaos and discord which have brought about profound personal and societal changes the Swede no longer can ignore. Simultaneously, the dinner party underscores the importance of a line repeated by Zuckerman several times during the novel, that no one ever truly understands the heart of another person; in Seymour's case, his wife's affair and Sheila's refusal to tell him about her shielding of his daughter after the bombing drive home the reality that his faith in the people around him has been completely misplaced, and they have instead completely betrayed him.