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They Do It With Mirrors is a work of detective fiction by Agatha Christie and first published in the US by Dodd, Mead and Company in 1952 under the title of Murder with Mirrors and in UK by the Collins Crime Club on November 17 in the same year under Christie's original title. The US edition retailed at $2.50  and the UK edition at ten shillings and sixpence (10/6). The book features her detective Miss Marple.

As the story opens, Jane Marple is paying a visit to her old friend Ruth Van Rydock. Miss Marple, Ruth, and Ruth's sister Carrie Louise were all friends together at the same school in Italy when they were girls. Ruth is worried that something is very wrong at Stonygates, the Victorian mansion where Carrie Louise lives with her husband Lewis Serrocold. She can't explain any real reason for these worries, but she fears that Carrie Louise may be in danger of some kind. Ruth asks Miss Marple to visit her and find out what is going on.

Carrie Louise is delighted to have Jane for a visit at Stonygates. The old Victorian mansion, though owned outright by Carrie Louise, has been converted into a home for delinquent boys which is run by Carrie Louise's husband, Lewis Serrocold. Lewis Serrocold is actually Carrie Louise's third husband; she was also once widowed and once divorced. Carrie Louise has always been attracted to men who had their minds on noble causes. Her first husband, Mr. Gulbrandsen, was a great philanthropist, and Mr. Serrocold is devoted to the idea of reforming juvenile delinquents and teaching them how to contribute to society. The boys are involved in theatrical productions and many other activities around the estate during the day, but at night they are confined to their own quarters. The family has the central block of the house to themselves.

The family includes many people who are connected to each other only through Carrie Louise. Mildred Strete is the only blood relative of Carrie Louise who is resident at Stonygates. She is Carrie Louise's daughter by her first marriage. Carrie Louise also had an adopted daughter, Pippa, who died after giving birth to her own daughter, Gina. Now an adult, Gina is married to an American named Walter Hudd and has recently returned to Stonygates. Juliet Bellever (nicknamed Jolly), a long time companion, caretaker, and friend of Carrie Louise is also a permanent fixture at the mansion. Stephen and Alex Restarick, Carrie Louise's stepsons from her second marriage, are also frequent visitors.

Also frequently present at Stonygates is Lewis Serrocold's assistant, Edgar Lawson. Edgar is an awkward young man whom the others dismiss as pompous and half-mad. He seems to suffer from both a persecution complex and delusions of grandeur. On several occasions he confides to others that he is the illegitimate son of a great man, and claims that powerful enemies are conspiring to keep him from his rightful position.

Christian Gulbrandsen, a member of the Stonygates Board of Trustees and the son of Carrie Louise's first husband from his previous marriage, arrives unexpectedly to see Lewis Serrocold. Everyone assumes he is there on business, but no one is sure exactly why. After dinner, Mr. Gulbrandsen retires to the guest room to type a letter. Miss Marple and the others gather in the Great Hall. A fuse blows out, and Walter goes to repair it.

Edgar Lawson bursts into the darkened room, screaming that Lewis Serrocold is his real father. Edgar and Mr. Serrocold go into the study and Edgar locks the door behind him. Everyone in the Great Hall listens intently as Edgar screams accusations at Mr. Serrocold, then they hear multiple gunshots. When the door is finally opened, they are surprised but relieved to see that Mr. Serrocold is alive and well, Edgar in tears, and several bullet holes in the walls.

Yet there has been a murder at Stonygates that night after all. When Juliet Bellever goes to check on Christian Gulbrandsen, she finds him dead. He was shot while working at his typewriter, and the letter he was writing is gone. Lewis Serrocold later reveals to the police that he took the letter to keep his wife from learning its contents. He explains that he and Mr. Gulbrandsen were both concerned that Carrie Louise's recent poor health was due to deliberate poisoning.

At that point, Alex Restarick, Stephen's brother, arrives. He becomes the most likely suspect since the police who come to investigate find an unaccounted period of time between his arrival in the car and his appearance in the Great Hall.

Alex Restarick's remarks about stage scenery lead Miss Marple to reflect on all kinds of stage illusion, such as conjurers who perform magic by using mirrors and stage sets and assistants who are in on the trick. When Alex and a boy who claimed to have seen something on the night of the murder are both killed, Miss Marple realizes who has been behind the plotting at Stonygates: Lewis Serrocold. The attempted poisoning of Carrie Louise never happened; it was an explanation hastily concocted by Serrocold to explain Christian Gulbrandsen's sudden appearance at Stonygates and his secretive conference with Mr. Serrocold. In fact, Gulbrandsen had discovered that Mr. Serrocold was embezzling from the Gulbrandsen Trust, and Serrocold and his unstable accomplice, Edgar Lawson, killed him to silence him. The murder was accomplished via illusion and misdirection, as Alex Restarick and Miss Marple both eventually realized: "behind the scenes" of the interior of the house, which everyone had been focused on, there was a terrace by which someone could exit the study and re-enter the house to commit murder without being seen by the rest of the residents. This was what Mr. Serrocold had done, while Lawson, using his acting talents and different voices, had continued both sides of the loud argument by himself.

When confronted by the police, Edgar Lawson panics and flees the house, jumping into an old boat in an attempt to cross a lake on the property. The boat is rotted though, and as it begins to sink, Lewis Serrocold jumps into the lake to rescue his accomplice. Both men are caught in the reeds that line the lake, and drown before police are able to rescue them, bringing an end to the case.