Brent is killed with an injection of potassium cyanide.

At this point, the order of events differs from the order in which they are revealed in the novel. They are revealed at the end of the book in a note found in a bottle and delivered to Scotland Yard. The following events will therefore act as a spoiler if you have not read to the end of the book.

Mr. Justice Wargrave enlists the help of Dr. Armstrong in what he says is a plan to catch the murderer. He pretends to be dead, and Dr. Armstrong proclaims that this is the case. None of the others look closely. They carry Wargrave up to his bedroom, and he is treated as another corpse.

That night, Wargrave meets Armstrong on the cliffs by prearrangement. He pushes Armstrong into the sea, where he drowns.

Wargrave drops a heavy marble clock on Blore's head, killing him.

Vera Claythorne, believing that she and Philip Lombard are the only people left alive on the island, surmises that Lombard must be the killer and shoots him.

When Vera returns to her room, she finds a noose there. Accepting the implied suggestion, she hangs herself.

Mr. Justice Wargrave writes out a full confession, admitting to all nine murders, in addition to that of the agent who acquired the island and made the arrangements, and announcing his own intention to commit suicide. He puts this note in a bottle and throws it into the sea.

Mr. Justice Wargrave kills himself, and the police find ten dead bodies on the island with no indication of who the killer could be. Senior officers at Scotland Yard discuss the impossibility of the crime.

Mr. Justice Wargrave's confession is picked up by a fishing trawler and sent to Scotland Yard.