Eleanor Tomlinson as Mary Durrant / O' Shaughnessy
Mary Argyle Durrant (birth name Mary O'Shaughnessy) is the eldest child of Leo and Rachel, about age 27, and the first child they adopted, while they were in New York City before World War II.
Mary's biological parents died when she was a child, and her aunt and uncle took her in. One day, she was running out of the tenement in which they stayed, and was knocked down by the Argyles' car.
Mary was bruised, but not hurt, by the accident. The Argyles took her to hospital to make sure there was no injury, and arranged for her to stay with them at their hotel for a few days.
Mary told the Argyles that she did not want to go home, and wanted to stay with them. They adopted her, and brought her back to Europe with them. According to Leo, she had shown acceptance of the benefits which they gave her, complacence, and enjoyment of all that was provided, but not love for her new adopted parents.
Mary herself remembers that she had wanted to continue enjoying all the luxuries that came with being rich, and all that was needed was a show of affection, so that the Argyles would adopt her. It was difficult for her, because she was not of an affectionate disposition, but she had managed it.
According to Kirsten Lindstrom, Mary is selfish and cold-hearted, and had never shown affection for anybody until she had married.
Mary had always felt "a faint contempt" for her adopted mother. She thinks that Mrs Argyle's insatiable mother love had something unnatural and animal about it. She is also of the opinion that Mrs Argyle was stupid to choose the adopted children that she did.
Mary is married to Philip. He calls her Polly, and he is the only person who does so, because the name is ludicrously inappropriate to her appearance.
Mary is described as being a "tall, serene-looking young woman". Her face is unlined, but she looks older than she actually is, which is probably because she has a "sedate maturity" about her. She has good looks, but "without a trace of glamour". Her features are regular, and she has good skin. Her eyes are of a "vivid blue", and her hair is fair. She wears her hair in a large bun at the back of her neck. She keeps to her own style, and is not influenced by what is fashionable.
Mary keeps her house very neat, as any kind of dust or disorder worries her.
Mary adores Philip, and sees him as her child, as well as her husband. He is slightly embarrassed by her possessive love, but Mary lacks the imagination to understand this. She also cannot understand why Philip craves the company of other people, and does not want only to be with her, as she wants only to be with him.
During the war, Mary had insisted on marrying Philip, although her mother had warned her not to. After Philip contracted polio, he was brought as a convalescent to Sunny Point. Mrs Argyle wanted them to stay there permanently, but Mary did not want to, as she wanted her home and her husband to herself.
At the time of Mrs Argyle's death, Mary and Philip were at Sunny Point for a visit, and were staying in the house. On the evening of the murder, they were in their room playing picquet. Mary had left the room between seven and seven-thirty, to go to the bathroom. However, as she had not mentioned this to the police at the time of the original investigation, Philip advises her not to mention it when the investigation is reopened.
Mary agrees with Philip's advice, but her agreement is placid and uninterested. She expresses no interest in knowing who really killed Mrs Argyle, because it is enough for her to know that it was not her or Philip. She tells Philip not to try to find out who the murderer is, and to leave everything alone.
Mary later catches Philip kissing Hester, and is very angry about it. When Philip tells her not to grudge him one kiss, because he had felt it would be fun to "be a gay dog again" and have a flirtation, she only tells him that his soup will get cold if he doesn't drink it. She later tells him that she wants him all to herself, and that she wants there to be nobody in the world but the two of them.
Philip later realises that there are two different sides of Mary. One is the gentle, reserved Mary, who stared at him with a puzzled frown when he teased her, and the other is a Mary who is passionate but incapable of affection, and to whom nobody matters except herself. Philip realises that even he only matters to Mary because he is hers.
At the end of the novel, Dr Calgary goes to Sunny Point to tell the family who murdered Mrs Argyle and Philip. Mary joins them in the library. She looks calm and composed, and is neatly dressed, but her face is "masklike in its lack of expression", and there is an air about her "as of a woman who walks in her sleep". She tells Dr Calgary that nothing he or anyone else can say will bring Philip back.
After Dr Calgary reveals who the murderer is, Mary springs at them like a tigress. Gwenda Vaughan and Dr Calgary hold her back.
Mary later says that she wishes she were dead. She tells the family that it was all Philip's own fault for wanting to stay at Sunny Point, trying to find out who killed Mrs Argyle, and that none of them understand.