The time told covers just under four days in the summer, from August 8 to August 11, except for the epilogue, which is slightly later. It is set in the present day. Except for the opening chapter and the epilogue, the entire action takes place on a deserted island off the coast of the county of Devon.
Ten people are lured to this island by letter or through an intermediary on some pretext. A judge (Lawrence Wargrave), an elderly unmarried woman (Emily Brent), a retired general (John Macarthur) and a young bon vivant (Anthony Marston) each receive an invitation, ostensibly from acquaintances, to vacation on the island or to renew old acquaintances. A young physical education teacher (Vera Claythorne) is to be there as a temporary secretary, an adventurer (Philip Lombard) is hired as an expert on dangerous situations, a doctor (Dr. Armstrong) is to watch the host's wife because of her problematic health, and an ex-police officer and owner of a detective agency (William Blore) is hired to keep an eye on her jewelry. A married couple (Thomas and Ethel Rogers) are hired as house staff to take care of the guests. The island is known to all from the press: An American millionaire has had a luxurious house built there, but has sold it to a mysterious stranger who usually appears in the invitations as a Mr. Owen, with the initials U. N.
The ten people arrive on the island in the boat of a local fisherman. They notice that in each room above the mantelpiece hangs a framed version of the well-known counting rhyme Zehn kleine Negerlein (in other versions: Indianerlein or Kriegerlein), in the course of which all ten protagonists perish. While having dinner together, they notice that ten porcelain figurines have been placed on a glass plate, corresponding to the figures in the counting rhyme. A little later, a ghostly voice sounds from a record, accusing all ten present of having caused the death of one or more people with impunity. It soon becomes clear that they cannot leave the island because there is no boat, and that the invitations were pretexts. The judge decodes the name U. N. Owen as "unknown", unknown.
The plot develops in the sequence exactly according to the counting rhyme: All ten characters perish one after the other, and the manner of death in each case can be compared with the specifications of the rhyme. In addition, one of the porcelain figurines disappears after each death.
On the first evening, August 8, Marston dies of cyanide in his drink, the next morning Ethel Rogers does not wake up (as it turns out later, due to sleeping pills poisoning with chloral hydrate). At first, the survivors still consider suicide or natural causes of death.
But on the second day, the boat that was supposed to bring daily supplies doesn't show up, and the butler Rogers notices that two figurines are missing. Lombard, Blore, and Armstrong become suspicious. They carefully search the island and house and discover that no other person is hiding on the island. At noon on the second day, Macarthur is murdered by a blow to the back of the head. Murder must now be assumed. At the same time, a storm breaks out, cutting the island off completely from the mainland. In a meeting of the survivors, Wargrave sums up the situation: The murderer must be among the remaining seven people, and none of them can be excluded from suspicion. In conversation, it becomes clear that there are many different theories among those who remain as to who the killer is and who can still be trusted. Likewise, it becomes apparent that the accusations of the record voice are essentially true.
On the morning of the third day, Rogers is found beaten to death in the woodshed, and in the morning Emily Brent dies of a cyanide injection. Wargrave now suggests a search of all rooms and persons for weapons and drugs. This suggestion is followed, but the pistol that Lombard brought to the island has disappeared. All dangerous items found are locked away. But later, Vera Claythorne suffers a shock when she feels a cold touch on her neck in her dark room. At her screams, the men rush over and find that it was just a piece of seaweed hanging there. Claythorne survives the shock unharmed, but Wargrave is missing. They find him in the drawing room, Armstrong diagnoses death by firing squad. In the night, Blore hears footsteps and makes sure Armstrong is not in his room. Blore and Lombard, whose revolver has reappeared, go to search but cannot find him. They conclude that Armstrong must be the murderer.
On the fourth day, the storm has passed, but the sea still goes so high that the island is unreachable. Blore, Claythorne, and Lombard spend the morning on the cliffs heliographing SOS signals toward the mainland. When Blore goes inside, he is struck by a block of marble. Claythorne and Lombard find a propelled floater: It's Armstrong. Vera Claythorne manages to take the revolver away from Lombard as they pull Armstrong ashore. She shoots him as he tries to retrieve the gun. When she returns to her room, there is a noose prepared on a hook on the ceiling with a chair underneath. She puts her head in the noose, kicks the chair away and hangs herself.
The epilogue of the novel consists of two parts. The first describes a meeting of the investigating police officers. This reveals that the inhabitants of the coastal village had been asked not to follow distress signals because it was a game. In any case, due to the swell, it was not possible to reach the island until the afternoon of August 12. Investigators can refer to diaries kept by some of those killed. Given the evidence, they consider it impossible that any of them could have been the perpetrator. So Armstrong's body has been pulled ashore, the revolver is found in the house and not with Lombard, the chair in Claythorne's room was found neatly placed against the wall. You're at a loss.
In the final pages, the denouement is given in the form of a message in a bottle. Wargrave confesses to the murders and details his actions. He had made Armstrong his accomplice on the third day by making him believe that a trap could be set for the murderer. With Armstrong's help, he faked his own death and was then able to move about the island. He pushed Armstrong over the cliff at night. He had finally shot himself last with Lombard's pistol, which, without obliterating Claythorne's fingerprints, he had attached to the doorknob with an elastic band. With the slackening of the muscles after death, the pistol would be flung away. That is how he would be found shot, as reported in the diaries of his victims.