Originally written for the stage and first performed in 1930, this three-act mystery brings Hercule Poirot into a country house thick with suspicion. Sir Claud Amory, a physicist working on a formula for a powerful new explosive, discovers that the papers have been stolen from his safe by someone under his own roof. He summons Poirot, then gathers his household to issue an ultimatum: when the lights go out, the thief may return the formula and face no questions. When the lights come back on, the formula is gone and Sir Claud is dead, poisoned in his after-dinner coffee. Poirot arrives with Captain Hastings and Inspector Japp only to find a murder already waiting for him. Confined within the house are the dead man's relatives, his secretary, and a smooth foreign visitor who may not be what he claims, each with a reason to want the papers and a reason to lie. Poirot applies his methodical attention to the small details others overlook, weighing motive against opportunity as the suspects circle one another. The drawing-room setting, the locked house, and the constrained cast of suspects make for a tightly contained puzzle. A later authorised prose novelisation by Charles Osborne, published in 1998, adapts the play for readers while preserving its structure and dialogue.