The novel is a standalone mystery in Agatha Christie's long-running series featuring Belgian detective Hercule Poirot. On his first night at the Solomon Hotel in Jerusalem, Poirot overhears a voice through his window say, "You do see, don't you, that she's got to be killed?" (1). He dismisses it as possibly relating to a literary collaboration but notes the speaker's nervous intensity.
The speakers are Raymond and Carol Boynton, young American siblings discussing their stepmother, Mrs. Boynton, a grotesque, domineering old woman who has kept her family in psychological imprisonment for years. Before marrying their father, the late Elmer Boynton, she worked as a prison wardress, or guard, and she exercises the same totalitarian control over her stepchildren and her own biological daughter, Ginevra, the youngest. Raymond argues that killing her is the only escape, particularly to protect Ginevra, who is nearing a mental health crisis. Carol is horrified but agrees, and Raymond claims to have a plan.
Sarah King, a young English doctor, meets Dr. Theodore Gerard, a distinguished French psychologist, in the hotel lounge. Together they observe the Boynton family: Lennox, the eldest son, sunk in apathy; his wife Nadine, calm but unhappy; Raymond and Carol, visibly tense; and Ginevra, sitting with a strange smile while secretly shredding a handkerchief. At the center sits Mrs. Boynton, whom Gerard perceives as radiating malevolence. He compares her to an animal tamer who has broken her charges' will.
Jefferson Cope, a well-meaning American and old friend of Nadine's, provides background. Elmer Boynton left all his money in trust to Mrs. Boynton for her lifetime, keeping the children financially dependent and isolated.
Sarah attempts to befriend the siblings. Carol breaks away at the Temple to apologize for Raymond's coldness, explaining that their mother forbids outside contact and describing their life as being "in prison" (39). Carol comes secretly to Sarah's room that night, but when she returns, Mrs. Boynton is waiting and extracts a promise that Carol will see Sarah no more. Meanwhile, Nadine privately begs Lennox to leave home with her; he refuses, admitting he lacks the courage.
On the Boyntons' departure day, Sarah impulsively confronts Mrs. Boynton, telling her she is pathetic rather than frightening. Mrs. Boynton responds: "I never forget. Remember that. I've never forgotten anything, not an action, not a name, not a face" (68). Sarah notices the old woman seems to look past her rather than at her. As the family leaves, Ginevra whispers to Gerard that she is secretly of royal blood and enemies are trying to kill her. Gerard recognizes these delusions of grandeur and persecution as symptoms of schizophrenia.
Sarah travels to Petra, the ancient city carved into rock in the Jordanian desert, with Gerard, Lady Westholme (a forceful English politician), and Miss Pierce (a timid, suggestible spinster). The Boyntons are already there. Raymond finds Sarah and declares his love, asking her to see past his cowed behavior around his mother.
After lunch the next day, Mrs. Boynton does something uncharacteristic: She tells her family to go walking without her. Gerard returns early to camp with malarial fever. The remaining group walks together in rare happiness. Sarah and Raymond drift off alone, but as the sun lowers, Raymond abruptly leaves, saying he must prove his courage.
Sarah returns to camp at six o'clock, passing Mrs. Boynton still motionless in her chair. At six-thirty, a servant reports the old lady appears ill. Sarah examines her and finds her dead, clearly for some time.
In Amman, Colonel Carbury, the local British authority, detains the party and consults Poirot. Gerard presents evidence: his hypodermic syringe disappeared and reappeared; digitoxin, a potent heart poison, is missing from his medicine case; and there is a puncture mark on Mrs. Boynton's wrist. Since Mrs. Boynton already took a digitalis-based heart medicine, an overdose could be mistaken for natural death. Poirot adds the overheard conversation and promises the truth within 24 hours.
Through interviews, Poirot uncovers contradictions. Sarah insists Mrs. Boynton had been dead at least an hour when examined at six-thirty, contradicting Raymond's claim of speaking to his living mother at 10 minutes to six. Lady Westholme describes an Arab servant who approached and angered Mrs. Boynton, detailing the servant's torn breeches and untidy puttees with striking precision despite a 200-yard distance. Nadine confesses she told Mrs. Boynton that afternoon she was leaving Lennox for Cope, fearing the shock killed her. Carol admits the Jerusalem plan but swears it was never carried out. Raymond insists his mother was alive but refuses to say more.
Poirot notes two glaring inconsistencies: If a family member committed the crime, they would have poisoned Mrs. Boynton's medicine rather than stealing a syringe; and the syringe theft suggests the killer lacked access to the medicine, pointing to an outsider. Miss Pierce provides a crucial detail: She saw one of the Boynton girls throw a hypodermic syringe into the stream the morning after the death, and Sarah claimed it.
At the final gathering, Poirot builds and dismantles cases against each family member, showing how Lennox, Raymond, and Carol each discovered Mrs. Boynton dead independently and, suspecting one another, created a web of concealment. Lennox confesses that in shock he mechanically wound her wrist watch. Raymond admits he lied to protect Carol. The second syringe is resolved: Raymond and Carol had stolen Nadine's syringe for their abandoned plan; Carol threw it away believing it incriminated Raymond; and Sarah claimed it to protect him. Poirot argues that Nadine's behavior also suggested prior knowledge of the death.
Poirot identifies the true killer: Lady Westholme. Her error was describing the Arab servant's clothing in detail impossible to see from 200 yards, because the "Arab" was Lady Westholme herself, disguised in riding breeches and a makeshift head-dress. She entered Gerard's tent for the digitoxin and syringe, injected Mrs. Boynton, changed back into her own clothes, and impressed her fabricated account on the suggestible Miss Pierce. Her motive: Before her marriage, Lady Westholme served a prison sentence under Mrs. Boynton's watch. Mrs. Boynton's recognition in Jerusalem, expressed in her words about never forgetting a face, threatened to destroy Lady Westholme's political career. Poirot states that her fingerprints can be compared to prison records. A gunshot rings out from Lady Westholme's room.
A newspaper reports Lady Westholme's death as a tragic accident. Five years later, Sarah, now married to Raymond, attends a London performance of
Hamlet in which Ginevra, treated by Gerard and now a celebrated actress, plays Ophelia. The family has healed: Lennox and Nadine have children, Carol has married Cope, and Raymond is a successful author. Ginevra expresses quiet pity for her mother, then softly recites lines from Shakespeare's
Cymbeline: "Fear no more the heat o' the sun."