This synopsis is based on the 1939 novel by Agatha Christie. Unmarked spoilers are contained within.
Eight strangers receive a message from the enigmatic Mr. U.N. Owen and his wife, who have invited the guests, for various reasons, to his house on the isolated Soldier Island just off the Devon coast. Upon arriving, the guests are informed that Mr. Owen's arrival has been delayed. Their surroundings are luxurious and modern, although each guest curiously notes the presence of ten porcelain figurines on the dining table and a copy of the nursery rhyme "Ten Little Soldier Boys" on the wall of each room. Over dinner, the guests—initially tense and awkward amongst themselves—warm up to each other. That all ends when a hidden gramophone plays a recording that charges each of the guests, as well as the two servants, with causing the death of another person:
- Dr. Edward Armstrong, a surgeon, is accused of causing the death of a patient by operating on her while drunk;
- William Henry Blore, a detective, is accused of lying in court and sending an innocent man to prison, where the man died due to frail health;
- Emily Brent, a deeply religious spinster, is accused of turning her maid Beatrice onto the streets when she became pregnant, which drove Beatrice to suicide;
- Vera Claythorne, a former governess turned gym teacher and secretary, is accused of murdering her previous charge by encouraging him to swim out to sea alone and drown, all so her lover Hugo could inherit the estate of the boy's father;
- Colonel Philip Lombard, a mercenary and adventurer, is accused of causing the death of twenty-one African natives by abandoning them and stealing supplies vital for their survival;
- General John Macarthur, a retired soldier, is accused of sending his wife's lover on a needless suicide mission during the First World War;
- Anthony Marston, a vain and self-absorbed playboy, is accused of recklessly running over two children with his car;
- Judge Lawrence Wargrave, a retired judge, is accused of steering a jury into sentencing an innocent man to death in spite of evidence supporting the accused man's innocence;
- and Thomas and Ethel Rogers, the butler and the cook, are accused of allowing their former employer to die of a preventable condition so they could gain her inheritance.
The guests are initially shocked and outraged at the accusations; they all either deny the charges or attempt to excuse them. But Lombard soon admits his guilt, and the self-absorbed Marston cares more about losing his license to drive than about the children he killed. When the guests compare the invitations they received, they soon realise that none of them have ever met Mr. Owen or his wife, and the messages summoning them to the island seem designed specifically to lure them in. They also deduce that U.N. Owen is likely a pseudonym, being a play on the word "unknown", and that Mr. Owen has sinister plans in store for his guests.
As the group agrees to leave the next day, Marston chokes to death after finishing his drink—a victim of cyanide poisoning. Several guests remark on how his death resembles the first line of the poem hanging in their rooms ("One choked his little self and then there were nine") and how one of the porcelain figurines has disappeared. Further tragedy arrives the next morning when Mrs. Rogers is found dead in her bed, having seemingly died in her sleep; another figurine has also disappeared, and the circumstances of her death also match a line of the poem ("One overslept himself and then there were eight"). General Macarthur, already haunted by the crime he committed, slips into a depression as he convinces himself that the group will never leave the island. Lombard convinces himself that Mr. Owen is hiding somewhere on the island, so he leads a search which ultimately proves that there is no place on the island where anyone could hide.
Any hope that the previous deaths might be unconnected are dashed when the group finds Macarthur, who had accepted his impending death, beaten to death ("One said he'd stay there and then there were seven"). A third figurine disappears to coincide with the latest death. Since the island cannot be concealing anyone else's presence, the guests figure out that Mr. Owen must be one of them—and that regardless of whoever Owen is, they are murdering the others. A storm soon hits the island, which prevents the remaining members of the group from leaving for the mainland; paranoia and suspicion soon sink in. Mr. Rogers, who has continued his duties despite the death of his wife, is slain with an axe while chopping wood ("One chopped himself in half and then there were six"), which leaves the guests to fend for themselves.
The group later finds Emily Brent dead from an injection of poison ("A bumblebee stung one and then there were five"), so suspicion falls on Dr. Armstrong. To try and resolve the situation, Wargrave suggests that the house be thoroughly searched and all dangerous objects locked away; during this process, however, the remaining guests discover that a revolver brought to the island by Lombard has vanished. The five remaining guests stay in the same room, with only one person permitted to leave at a time. While leaving to take a bath, Vera is startled by a clump of seaweed rigged in her room and screams, summoning the men to her aid. Once the situation calms down, they realise Wargrave has not joined them. The remaining four guests find him prone and dressed in a mockery of a judge's robes ("One joined Chauncery and then there were four"), and Armstrong pronounces him dead of a gunshot wound. Lombard later finds the gun returned to his room.
That night, Armstrong slips out of the house, all but confirming that is the murderer. The remaining guests attempt to find him, but their search turns up nothing—he has disappeared without a trace. The last three guests, now overcome with paranoia, refuse to go back into the house, so they attempt to signal the shoreline for help. When Blore returns to get food, he is struck and killed by a large clock shaped like a bear that someone has dropped from a window ("A big bear hugged one and then there were two"). After Blore's death, Vera and Lombard discover Armstrong's body washed up on a beach; as Armstrong had been killed before Blore, he was a false lead ("One was eaten by a Red Herring, and then there were three").
Vera and Lombard have now convinced themselves that the other is the murderer. While moving Armstrong's body, Vera grabs Lombard's revolver, and when he tries to take it back, she shoots him and leaves him on the beach ("One frizzled up in the sun and then there was one"). Vera expresses relief at being the last survivor, but her sanity has been shattered by paranoia, stress, fear, and guilt. She hallucinates that she is being watched by Cyril, the boy she manipulated into drowning, and her former lover Hugo, who realised what she had done and rejected her. After returning to her room, she discovers that someone has set up a noose. In a post-traumatic daze, she decides to complete the nursery rhyme: "One went and hanged himself, and then there were none."
When the storm lets up and a group of rescuers and police officers reach the island, they discover ten murder victims, but no apparent murderer. No one could have left the island during the storm, so the killer would presumably still be on the island. But each of the victims—including Vera—are found in a way that suggests that none of them could have killed the others and then themselves. The scene leaves everyone baffled.
An indeterminate amount of time after the murders, someone finds a message in a bottle—which turns out to be a confession from Lawrence Wargrave. From an early age, Wargrave realised that he had sadistic and murderous impulses, though they were balanced by an desire to seek justice and protect the innocent. Upon receiving a diagnosis for a terminal illness, he decided to collect a group of people who had committed murders that could not be punished by law and deliver justice to them himself. He framed the murders around the nursery rhyme for his own amusement. He faked his death with the help of Armstrong—who was convinced he was innocent—then manipulated events to ensure the deaths of the remaining four guests. And to ensure his own death, complete the rhyme, and deliver justice upon himself (as he was now a murderer), he rigged a contraption to shoot himself in such a way that would make investigators believe he had been shot by someone else. Wargrave admits that he likes the idea of an unsolvable murder, but has written his confession in the hopes that someone will one day know what he has done.