Lydia Garth, a blind nun, recalls the events that led to her taking holy orders:

Waiting at a London airport for the Copenhagen plane on which her employer, Dr Simon Blake, is returning from a conference, she is troubled by blurred vision. At the bookstall, she meets a Frenchman, Paul Devandier. When Blake arrives it is revealed that he knows them both, and he arranges for the three of them to have dinner that evening. Paul and Lydia continue seeing each other, and by the time he has to return to France they have agreed to marry. But after he leaves Lydia suddenly goes blind. Unwilling to risk an operation that could kill her or leave her brain damaged, she decides to return to the nunnery in which she had been educated and take her vows.

Back at the nunnery, the Reverend Mother tells her that she thinks she is unhappy and should return to her life outside, rather than attempt to run away. Lydia visits Simon, who has arranged for Paul to be present. Leaving them alone Paul reaffirms his love for Lydia and, over her misgivings, insists that they marry.

They move to his house in France, where they are greeted by the butler, Joseph, Paul's mother, father and sister-in-law, and Veritée, their nearest neighbour and long-standing family friend. That evening, when Paul leaves the room for a moment, Veritée appears at the window, her face rigid with hatred. Lydia senses a presence but Veritée does not answer when she calls out, and leaves unseen when Paul returns.

Next day, Lydia is introduced to Max ffoliot, an English artist who lives in the village, and they strike up a friendship. Etienne, Paul's father, takes her on a tour of the house, pointing out a portrait of an ancestor who chopped off his withered hand rather than live with a physical defect. After she leaves the room Veritée appears, and in her conversation with Etienne the two of them reveal that they are conspiring to destroy the marriage so that Veritée can marry Paul.

At a party to celebrate the marriage, Veritée moves Lydia's glass so that she spills it when attempting to return a toast proposed to her. Joseph sees her, and offers his silence and collaboration in exchange for sexual favours.

The following day Paul, Lydia, Veritée and Max go swimming. Unseen, Veritée deliberately steers Lydia into strong currents and ignores her pleas for assistance. Paul appears, forcing Veritée to rescue her.

While Paul is in Paris, Lydia asks Max if he will write a love letter to Paul to her dictation, but the letter is not finished and Joseph overhears the conversation. That evening Veritée brings Lydia a telegram from Simon asking her to return to London. While in her room she steals the letter to Paul, and gives it to Etienne.

Paul returns and accuses Lydia of having an affair with Max, assuming that the unsigned letter is to her. She furiously tells him the truth, and reveals that she is pregnant. Overhearing Etienne conspiring with a second person (who is, in fact, Veritée) she falls down some steps and loses her baby.

After recovering, she leaves the chateau while Paul is in the village. When he returns, Etienne tells him she has gone to England and that it would be useless to try and find her. In London she keeps her appointment with Simon, who has found a surgeon who thinks he can cure her. The operation is a complete success.

She returns at night to the chateau and finds that Paul is not there. Veritée attempts to kill her by leading her through an open window. She realises she has been exposed when she discovers that Lydia can now see. Lydia then confronts Etienne, who is ironically now confined to a wheelchair. He admits that he did not want Paul to marry a blind woman. Lydia rushes out to look for Paul, pursued by Joseph and Veritée, but Veritée drives the car over a cliff and both are killed. Via a series of coincidences Paul and Lydia are reunited at the airport bookstall where they had first met.