Alan Desland, Rupert Frazer is an English antique dealer who specializes in ceramics. A solitary man, he travels to Copenhagen on business and hires a German-born secretary, Karin Foster Meg Tilly, fluent in English, Danish, and German. Alan's attraction is immediate, and he falls deeply in love with her. Karin displays a quiet sensitivity—she cries during a classical concert—alongside a dispassionate coldness, such as when she breaks the neck of an injured seagull. When Alan casually remarks that he might have trouble marrying a woman with a child, she becomes clearly upset. Before leaving Copenhagen, Karin expresses her love for Alan, and they agree to marry.
Alan's mother in England expresses concern that they know little about Karin's background. Alan begins to notice Karin's strange behavior, including a fear of the sound of children and a fearfulness of the dark. Karin refuses to marry in a church, and on a holiday to the United States they wed in a civil ceremony in Florida. While swimming in a lake on their honeymoon, Karin sees a body beneath the water that Alan identifies as a log.
Returning to England, Karin charms his social circle during discussions of philosophy and religion. Alan is troubled when he asks his best friend, a vicar, if all things can be forgiven, a moment in which Nicholas Le Prevost the vicar becomes part of the conversation. Karin wins a rare statuette of a girl in a swing at auction, which Alan authenticates as valued over £200,000. The impotence he experienced on their wedding night gives way to unbridled sexuality. In conversations with Alan and the vicar, Karin explores the connection between spiritual love and physical love, which she believes is absent in Christianity but embraced by pagan cults.
Alan's joy at hearing of Karin's pregnancy is offset by strange events and apparitions. He has visions of a green tortoise toy and they both hear a child's voice on the phone. After deciding to receive Holy Communion, Karin is disturbed by the vicar's sermon on the commandment against killing. At the communion railing, she takes the eucharist in her hand, but does not receive it, and collapses. Alan's reassurance that whatever's past is past prompts Karin to reply
> Nothing is past.
Karin and Alan both hear a child crying in the garden and find a doll face down in the water fountain. Alan shuts all the doors and windows and draws the drapes, yet the cries continue in the garden during a violent storm. Alan sees the green tortoise toy again and the sense that something unresolved lingers around them.
After the storm Karin asks Alan to take her away. Before leaving he discovers a receipt for a green tortoise toy, and concludes she bought it for a daughter predating their marriage, surmising she killed the girl out of fear that Alan would reject her with a child. Realizing the damage caused by his "careless words," Alan cries out, "May God have mercy." They drive to the beach, where she walks into the surf, pours water over her head in a gesture of baptism, and tells her he knows what she did. She takes her clothes off, hands him her wedding ring, and they make love on the beach. As the waves roll over them, she faints in his arms.
At the hospital, Leonard Maguire the doctor confirms she had an ectopic pregnancy, and that she had previously given birth. The next day he's told she died during the night. At an inquest hearing Alan sees an apparition of Karin in a hooded cloak at the back of the courtroom and breaks down in tears. Haunted by his careless words, Alan understands that his need for a tidy life resulted in the tragedy. When he returns home, he hears Karin in the garden, and walks out to find her on the swing.
