Ron and Priscilla are college sweethearts who meet at Harvard in the fifties. They spend a good amount of time in bed together, though Priscilla insists on maintaining her virtue. For her boyfriend's sake, she is willing to put on "one woman parades," wherein she displays herself naked for a minute, "mock model style."

One day, Priscilla reveals to Ron that she is a twin. Her sister attends the University of Chicago. After this revelation, Ron "could not forget it, or quite forgive her." Indeed, "the monstrous idea flirted at the back of his bead that she was half a person; there was something withheld, something hollow-backed and tinny about the figure cut in his mind even as their courtship proceeded smoothly toward marriage."

Despite her rich, Episcopalian background — and his poor, Baptist one — they do indeed get married. Ron meets Priscilla's twin, Susan, for the first time at the wedding. Susan is tan, short-haired, and languid in a "West Coast manner." To her parents' dismay, she's going to study art history at the graduate level at U.C.L.A.

There's a strong tension that manifests in a certain stiffness between Susan and Rob. Susan is in Rob's head when he and Priscilla consummate their marriage, "exciting him, urging him on through Priscilla's pain." It was a "botch of defloration" — one that "put an unfortunate crimp in this infant marriage."

Rob attends Yale Law school, becomes a specialist in tax law, and ultimately settles with his wife in Greenwich. They have two girls and a boy. Susan marries an jovial, artsy man named Job, and they "keep pace" in California with two boys and a girl. The two families are friendly and often visit each other. On one occasion in the hot tub, Rob sees Susan naked for the first time; he's fascinated by her body and how it diverges from her sister's.

Susan's marriage on the West Coast starts to fail. She visits Rob and Priscilla more and more often; they notice that she often fails to eat and sleep. "Aren't you a woman, or are you only part of marriage?" Priscilla asks her sister — a comment that makes Rob worry she still hasn't forgiven him for the way he consummated their marriage.

Rob can't work up the courage to ask Susan about the details of her marriage. He asks his wife instead, and she explains, "Jeb's a bastard....My parents knew it, but what could they do? She had to get married, once I did. And all men are bastards, more or less."

Susan and Job divorce after the 1970s recession, and Priscilla and Rob face similar marital peril. They both have affairs, and Priscilla accuses Rob of never having loved her. "You just loved the idea of sneaking into a family," she tells him.

The two divorce a year later after Rob is fired from his job for directing clients into a faulty tax shelter. He takes a new position in LA, where he partakes in the local debauchery to "even the score" with his former wife. One day at the mall he runs into his niece, who passes on Susan's contact information.

The pair exchange letters and he flies to meet her in the Bay Area. She's thin and casually chic, with long grey hair. "He had never seen Priscilla look like this," Rob reflects. "Susan's shininess was exciting; he wanted to seize her before she dwindled away entirely."

After dinner, she asks him, "Do you see me? Me, I mean."

"Who else?" he replies. "I've always liked you. Loved, should I say?"

They acknowledge that things between them are "complicated," and Susan insists she doesn't want to be "just a way of correcting a mistake."

"Why not?" Rob replies, with "faith that in an affluent nation a need, honestly confessed, has a good chance of being met.

Given that it's the eighties, Susan is "nervous about herpes" and asks to wait to have sex. In the meanwhile, she gives Rob a naked parade like her sister used to give him in his dorm room. The "delay Susan imposed...helped Rob grasp the blissful truth that she was only another woman."