The novel is primarily set in a large mansion on Soldier Island, a fictional small island off the coast of Devon in South West England. Because the island is only accessible by boat, a storm prevents the principal cast of characters from leaving the island, even as an unknown murderer picks them off one by one. While Christie’s murder mysteries are often associated with antiquated old buildings full of hiding places and hidden passageways, she sets this novel in a notably modern house. When some male guests attempt to search the premises for a hidden intruder, the narrator presents a detailed description of the setting:
The house was easily searched. They went through the few out-buildings first and then turned their attention to the building itself. Mrs. Rogers's yard measure discovered in the kitchen dresser assisted them. But there were no hidden spaces left unaccounted for. Everything was plain and straightforward, a modern structure devoid of concealments. They went through the ground floor first. As they mounted to the bedroom floor, they saw through the landing window Rogers carrying out a tray of cocktails to the terrace.
The mansion is a “modern structure devoid of concealments” instead of a spooky old house full of places to hide or conceal a weapon. The “plain and straightforward” nature of the setting contributes to the sense of mystery, as it appears to be an unlikely place to plan an intricate series of murders. Despite the grave circumstances, the guests attempt to maintain a sense of normalcy in the mansion at first, and the butler, Thomas Rogers, continues to serve drinks to the guests even after the death of his wife, Ethel.