The first person narrator is Charles Hayward who, towards the end of the Second World War, occupies some post in Cairo. There he meets Sophia Leonides, who works for the Foreign Office there. They fall in love with each other but put off getting engaged until after the end of the war when they will be reunited back home in England.
When Hayward arrives back home it is only to find an obituary in The Times: Sophia's grandfather, Aristide Leonides, has died, aged 87. A Greek by birth who came to England in 1884 at the age of 24, he had become a very successful entrepreneur and the owner of a large number of restaurants. His first marriage, at the time considered a misalliance, ended abruptly in 1905 when his wife Marcia died of pneumonia. More than 30 years later, in 1937, aged 77, he married Brenda, a young girl of 24 who had been a waitress in one of his many restaurants. The whole Leonides family strongly disapproved of the marriage, of course suspecting the second Mrs Leonides to be nothing but a scheming person just waiting to inherit a fortune. Strangely, due to the war, the whole family has been living together under one roof, in a large house near London (referred to by the narrator as "Crooked House").
There is nothing surprising about a 87 year-old man dying. It turns out, however, that Aristide Leonides has been poisoned. A diabetic, he was given one of his regular injections by his young wife. But someone had substituted the insulin for his eye-drops, which, once injected into Leonides's bloodstream, resulted in imminent death.
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The obvious suspects are Brenda Leonides herself—no longer willing to wait for her inheritance—and Laurence Brown, a conscientious objector who also lives in the house as private tutor to Eustace and Josephine, Sophia's younger brother and sister. Rumour has it that Brenda Leonides and Brown have been carrying on an illicit love affair right under old Leonides's nose. In fact all family members want them to be the murderers because it would be the easiest way out for them and the family's good name would not suffer. Soon the police find out two things: (a) that anyone could have committed the crime, as there are no locked doors in the huge house; and (b) that no one really has a motive: Aristide Leonides was a very generous man, who had given away a considerable portion of his money to the other members of his family during his lifetime.
Charles Hayward's introduction to the household is rather unconventional, his position somewhat hybrid: On the one hand, he wants to marry Sophia (and vice versa); on the other, he has agreed to help his father, Assistant Commissioner of Scotland Yard, investigate the crime. For the time being, these investigations do not lead anywhere: All family members are interviewed several times, but there is no clue whatsoever as to the identity of the murderer.
The Leonides family are a strange lot: Edith de Haviland, Aristide's unmarried sister-in-law, came to stay with him after his first wife's death in order to supervise his children's upbringing. Now she is a woman of 70 who, as it turns out only at the end, is fatally ill. She may or may not have been in love with her brother-in-law when they were both younger. Roger, the eldest son and always Aristide's favourite, is a failure as businessman and has steered the catering business bestowed to him by his father to the brink of bankruptcy; his wife Clemency, an austere-looking scientist, has never been able to enjoy the wealth offered by her husband's family. Philip, Roger's younger brother, has suffered all his life under his father's preference for Roger and retreated into a distant world of books and bygone historical epochs, spending all his waking hours in the library of the house. Philip's wife Magda is a modestly successful actress to whom everything, even a murder in the family, is a stage show in which she wants to play a leading part. Aristide Leonides, who was a particularly good judge of character, has redrafted his will and decided to bequeath his whole fortune to his granddaughter Sophia because she is the only one to have inherited his shrewd sense for managing money. Sophia knows all about it, whereas the other family members have been deluded into thinking that they will get a fair share of the old man's money. 16 year-old Eustace still suffers from the aftereffects of some childhood disease, but otherwise is just an ordinary boy. His sister Josephine, aged 12, on the other hand, is exceptionally ugly to look at and preoccupied with detective stories, which she has been devouring for some time.
It is Josephine on whose life an attempt is made by means of a marble door-stop placed on top of a door. She is hospitalized but recovers quickly from her head injuries. Everybody is under the impression that the killer has tried to get rid of Josephine because she has been bragging about knowing the identity of the murderer and having written down everything in her little black notebook. When Hayward finds a batch of love letters from Brenda Leonides to Laurence Brown hidden away in the attic the two are arrested.
The police are not convinced that they have found the real perpetrators though. Still the Leonides family are glad the nightmare is over, even if they are not sure about the two alleged lovers' guilt themselves, and they go to great lengths to provide the best lawyers for them. Then an unexpected thing happens which clearly shows everybody that the nightmare is not over yet: Janet Rowe, the old nanny, dies after drinking a poisoned cup of cocoa originally intended for Josephine (who has by now returned from the hospital). Everyone thinks Josephine is in great danger now, and her grandmother, Miss de Haviland, takes her out in the car to have an ice-cream, obviously just to get her away from the house. However, they never return: They are both found dead somewhere near a quarry. It turns out in the end that Josephine is the murderess ("Today I killed grandfather."), that she even staged an attempt on her own life in order to draw suspicion away from her. Edith de Haviland, herself critically ill, has more or less known all along what has really been going on and, by killing Josephine (and then herself) spared her the ordeal of standing a murder trial.
Now that all suspicions have been swept away, Sophia Leonides and Charles Hayward can safely get married.
There is also a short story by science fiction author Robert A. Heinlein entitled "And He Built a Crooked House".Outline of the plot