The book is a memoir of Maureen Johnson Smith Long, mother, lover, and eventual wife of Lazarus Long. Maureen is ostensibly recording the events of the book while held in prison alongside Pixel, the eponymous character of The Cat Who Walks Through Walls.
Maureen, born on July 4, 1882, recounts her girlhood in backcountry Missouri, discovery that her family is a member of the long-lived Howard Families (whose backstory is revealed in Methuselah's Children), marriage to Brian Smith, another member of that group, and her life—largely in Kansas City—until her apparent death in 1982. In addition, Maureen lives through, and gives her (sometimes contradictory) viewpoints on many events in other Heinlein stories, most notably the 1917 visit from the future by "Ted Bronson" (Lazarus Long), told from Long's point of view in Time Enough for Love, D. D. Harriman's space program from The Man Who Sold the Moon, and the rolling roads from The Roads Must Roll.
Maureen's adventures include a series of sexual encounters, beginning in childhood wherein, having just had her first sexual intercourse, she is examined by her father, a doctor, and finds herself desiring him sexually. Her story then encompasses various boys, her husband, ministers, other women's husbands, boyfriends, swinging sessions, and the adult Lazarus Long/Theodore Bronson. Additionally, she continues a lifelong pursuit of her father sexually, encourages her husband to have sexual intercourse with their daughters, and accompanies him when he does; but forbids a son and daughter of hers from continuing an incestuous relationship, primarily for the sister's reluctance to share the brother with other women. All of these are set against a history lesson of an alternate 20th century in which a variety of social and philosophical commentary is delivered.
She is eventually rescued from prison by Lazarus Long and other characters of various novels in the ship Gay Deceiver (from The Number of the Beast), and after rescuing her father from certain death in the Battle of Britain, is united with her descendants in a massive group marriage in the settlement of Boondock, on the planet Tertius. Maureen ends her memoir and the Lazarus Long saga with the phrase "And we all lived happily ever after".