Thirteen guests gather for cocktails at the Cornish home of Sir Charles Cartwright, a celebrated actor lately retired from the stage. Among the local and London friends is the mild Reverend Stephen Babbington, who takes a sip of his drink and collapses, dead within minutes. There is no trace of poison in the glass, no apparent motive, and seemingly no reason anyone would want to harm so harmless a man. The death is written off as natural until, weeks later, a respected Harley Street physician dies in almost identical circumstances at his own house party. Sir Charles, unable to let the matter rest, takes up the investigation himself, joined by the quietly observant Mr Satterthwaite and the determined young Hermione Lytton Gore, known to everyone as Egg. Hercule Poirot, present at that first fatal evening, watches the amateurs work and waits. Cast in the form of a stage play and divided into three acts, the story follows the small society of suspects as suspicion hardens into certainty and a pattern emerges from two deaths that should have nothing in common. The retired actor's instinct for performance and disguise proves as useful as Poirot's logic, and the question is not only who killed two unremarkable men, but why anyone would bother. The answer turns on a single overlooked detail, and on the difference between the part a person plays and the person beneath it.