Dr. Armstrong's drowning

Dr. Armstrong's drowning is the pivotal event in which the murderer, Lawrence John Wargrave, lures the unsuspecting physician to the cliff edge under the pretense of a shared plan to trap the killer, then pushes him into the sea, thereby eliminating the one man who could expose the judge's staged death and reducing the number of living guests to three.

Background and the Deceptive Alliance

After the death of Emily Brent, Wargrave had selected Dr. Armstrong as his unwitting ally. The judge, knowing that a rigorous search of persons and effects was imminent, had already hidden the revolver and disposed of his remaining poisons. He then intimated to Armstrong that they must carry out a plan: Wargrave would appear to be the next victim, which might rattle the murderer, and once he was supposed to be dead he could move about the house and spy upon the unknown killer. Armstrong, a gullible man who knew Wargrave by sight and reputation, found it inconceivable that a man of the judge's standing could actually be a murderer; all his suspicions were directed against Philip Lombard, and Wargrave pretended to concur. The doctor was keen on the idea.

The Staged Death and the Rendezvous

That evening, Wargrave carried out the deception. Using a little plaster of red mud on his forehead, the red curtain and the grey wool, he set the stage in the drawing-room. The flickering candlelight made the illusion convincing, and only Armstrong examined him closely. When Vera Claythorne screamed over the seaweed that Wargrave had arranged in her room, everyone rushed upstairs, and the judge took up his pose as a murdered man. Armstrong acted his part in the most professional manner, and they carried Wargrave to his bed. No one worried about him, all being too terrified of each other. Wargrave had a rendezvous with Armstrong outside the house at a quarter to two. He took the doctor up a little way behind the house on the edge of the cliff, saying that from there they could see if anyone approached and would not be seen from the house as the bedrooms faced the other way. Armstrong was still quite unsuspicious.

The Murder on the Cliff

At the cliff edge, Wargrave uttered an exclamation, leaned over, and told Armstrong to look—wasn't that the mouth of a cave? The doctor leaned right over. With a quick vigorous push, Wargrave sent him off balance and splashing into the heaving sea below. Armstrong, who had been a red herring in the nursery rhyme's scheme, was swallowed by the water. Wargrave returned to the house; it must have been his footfall that William Henry Blore heard. A few minutes after returning to Armstrong's room, Wargrave left it, this time making a certain amount of noise so that someone should hear him. As he got to the bottom of the stairs, he heard a door open; they must have just glimpsed his figure as he went out of the front door. He went straight around the house and in at the dining-room window, which he had left open, then shut it and later broke the glass. He then went upstairs and laid himself out again on his bed.

Discovery and Aftermath

Armstrong's body was not found until the following day. Vera Claythorne and Philip Lombard, searching the shoreline, spotted a bundle of clothes wedged between two rocks, flung there by the tide. They scrambled over and found a purple discolored face—a hideous drowned face. It was Armstrong. The discovery forced the two survivors to realize that there was no one on the island except themselves, and that one of them must be the killer. The police later determined that Armstrong must have gone into the sea sometime during the night of August 10th–11th; his body was deposited at high water on the 11th around 11 a.m. Crucially, the body had been dragged above high water mark and laid out straight on the ground, proving that someone was alive on the island after Armstrong was dead. The drowning, corresponding to the nursery rhyme's line "A red herring swallowed one," was the final piece of Wargrave's grand design, clearing the way for the last three deaths.