Eight people arrive on a small, isolated island off the Devon coast, each having received an unexpected personal invitation. They are met by the butler and cook/housekeeper, Thomas and Ethel Rogers, who explain that their hosts, Mr and Mrs Owen, have not yet arrived.
A framed copy of an old rhyme hangs in every guest's room, and on the dining room table sit ten figurines. After supper, a phonograph record is played; the recording accuses all ten people present of having committed murder, then asks if any of the "prisoners at the bar" wishes to offer a defence.
No-one knows their hosts, and Mr Justice Wargrave suggests that the name given, "U N Owen", is a play on "Unknown". Marston finishes his drink and promptly dies of cyanide poisoning. Dr Armstrong confirms that there was no cyanide in the other drinks and suggests that Marston must have poisoned himself.
The next morning, Mrs Rogers is found dead in her bed. Suspecting that a murderous U N Owen may be hiding on the island, the guests search but find nothing. After General MacArthur dies from a heavy blow to the head, it becomes clear that one of the seven remaining persons must be the killer. The following day, Mr Rogers is found dead at the woodpile, and Emily Brent is found dead in the drawing room, having been injected with potassium cyanide. The guests realise that one of the figurines in the dining room is being removed after each death, and that the manner of their deaths corresponds with the wording of the rhyme.
Wargrave suggests searching the rooms, and Lombard's gun is found to be missing. Vera Claythorne goes up to her room and screams when she finds seaweed hanging from the ceiling. Most of the remaining guests rush upstairs; when they return they find Wargrave still downstairs in his chair, crudely dressed in the attire of a judge. Dr Armstrong pronounces him dead from a gunshot wound to the forehead.
That night, Lombard's gun is returned, and Blore sees someone leaving the house. Armstrong is absent, and Lombard and Blore unsuccessfully search for him. Vera, Blore, and Lombard decide to go out together the next morning. When Blore returns for food, he is killed by a bear-shaped marble clock that falls from Vera's window sill. Vera and Lombard find Armstrong's body washed up on the beach, and each concludes the other must be responsible. Vera grabs Lombard's gun and shoots him dead.
Vera returns to the house in a shaken, post-traumatic state. She finds a noose and chair arranged in her room. Overcome by guilt, she hangs herself in accordance with the last line of the rhyme.
Scotland Yard officials arrive to find nobody alive. They discover that a sleazy lawyer and drug trafficker called Isaac Morris had arranged the invitations and ordered the recording. However, he had died of a barbiturate overdose on the night the guests arrived. The police reconstruct the deaths with the help of the victims' diaries and a coroner's report. They are able to eliminate several suspects, but cannot identify the killer.
Much later, a trawler hauls up in its nets a bottle containing a written confession. In it, Mr Justice Wargrave recounts that all his life he had had two contradictory impulses: a strong sense of justice and a savage bloodlust. He had satisfied both through his profession as a criminal judge, sentencing murderers to death following their trial. After receiving a diagnosis of a terminal illness, he had decided on a private scheme to deal with a group of people he considered to have escaped justice.
To set up his scheme, Wargrave had employed an agent, Morris, to purchase the island and to make the arrangements, before killing him with a lethal dose of barbiturates. Posing as one of the guests, he had with Dr Armstrong's naive assistance faked his own death on the pretext that it would help the group identify the killer. After disposing of Armstrong and the remaining guests, and moving objects to confuse the police, he finally killed himself using the gun and some elastic to ensure that his true death matched the account of his staged death recorded in the guests' diaries. Wargrave had written his confession and thrown it into the sea in a bottle in response to his "pitiful human need" for recognition.
