Dr David Henry delivers the twins his wife, Norah, is carrying, in an emergency procedure on March 6, 1964, with the help of his nurse, Caroline. The first born twin is a boy they name Paul, the second, a little girl whom they call Phoebe. David notices that Phoebe has Down Syndrome, which he knows means that Phoebe might have a heart defect that will cause her to die in childhood, or young adulthood. He had a younger sister, June, who had passed away at the age of twelve and he does not think that he can go through a trauma like that again. David decides there and then that he will give Phoebe up and that she will be institutionalized. He gives her to Caroline and tells her to take her to the facility he has selected, but she doesn't like it at all and cannot imagine leaving the baby there. Instead, she decides to keep Phoebe and raise her as her own.
They get off to an inauspicious start. Caroline's car breaks down whilst she is driving to the store in the stow, and she is stranded with her new baby.A truck driver, Al, comes to their aid and drives them back to Caroline's home. There is a spark between them that Caroline is looking forward to exploring further.
David, meanwhile, has not told Norah about Phoebe, except to say that she died at birth. He learns that Caroline has kept her and subsequently that they have moved to Pittsburgh to start over.
A year later and David and Norah have still not come to terms with the "death" of their daughter. They move home but it doesn't seem to help; they are not attracted to each other anymore and the romance has gone out of their marriage. David sends money to Caroline for Phoebe, and pretends to himself that he is trying to find them, but the extent of his effort is a few letters. Caroline, one the other hand, seems to be thriving. Al has managed to track her down in Pittsburgh and the two are beginning a romance.
Five years later, the Henry's are further apart than ever, and appear no more than strangers who share the same last name. David has started a photography business and has his own darkroom, which is convenient, because Norah never goes down there and so he can keep the letters and pictures of Phoebe that Caroline sends him without fear of Norah ever finding them. He finds constant fault with Paul, angry with him for not being Phoebe, which makes Norah very protective of him, and distances them from each other further.
Phoebe, meanwhile is doing very well in Pittsburgh and despite her disability is ready to enter the mainstream school system. Caroline sees that Al has sincere intentions towards her, but wonders if he pretends to feel more for Phoebe than he really does, because he wants to marry Caroline and wants to impress her with his love for her child. When Phoebe has an allergic reaction to a bee sting, Al drives them to the hospital and reprimands a nurse who disparages Phoebe because of her Down syndrome. This is an epiphany for Caroline who realizes that he really does love Phoebe as much as he says he does. She accepts Al's proposal of marriage knowing that he will take good care of both Phoebe and herself.
When Phoebe and Paul turn thirteen years old, their home lives could not be more different. Phoebe is being raised in a happy home by two parents who love each other, and shower her with love and encouragement. Paul is finding that impressing his father comes at the detriment to himself; he is a talented musician and wants to study guitar at Juilliard, but David wants him to choose a career that is more traditional and reliable. David and Norah are living separate lives, and fight a lot, particularly about Paul and his own dreams for his future. Norah flees the house whenever she can and on vacation in Aruba she has an affair, which David blames himself for, but does nothing to change his behavior to prevent her from having another.
By the time the twins are twenty five years old, Norah and David have divorced. Paul has a girlfriend of his own, and feeling pressured by his father to conform to something that he cannot, leaves for France where the pair travel around and study music. Phoebe is also in a relationship with a young man who also has Down syndrome. She wants to marry him and wants them to live together in a group home but Caroline is nervous about letting her move out.
David thinks about telling Norah everything but decides against it. He dies from a heart attack and after his death Norah sorts through the papers and pictures hidden away in the darkroom. She finds the correspondence about Phoebe, and begins to know her husband better after his death than she ever did when they were married. Caroline brings Phoebe to meet Norah and Paul, and the twins are immediately close as if they had grown up together. Paul drives Phoebe to visit David's grave, and wonders how their lives would have turned out if they had been raised together. It is suggested by the narrative that their lives will forever intertwine.
