A masquerade at the height of the season ends in scandal when a young viscount turns up stabbed through the heart in a supper room, his body oddly rigid and his fists still clenched. He and five companions had come dressed as the figures of the commedia dell'arte: Harlequin, Columbine, Pierrot, Pierette, and the two Punchinellos. By morning a second member of the party, an actress rumoured to be engaged to the dead man, is also gone, killed by an overdose of cocaine. The police see a straightforward case, but the small details refuse to settle. Hercule Poirot, observing from a distance with his usual interest in order and pattern, fixes on the curious matter of the costumes themselves and on the precise sequence of who was seen where, and when. Narrated by Captain Hastings, the case turns on timing, impersonation, and the gap between what witnesses believe they saw and what actually occurred. Across a tight, fast-moving frame, Poirot reconstructs the night through reasoning rather than legwork, weighing motive against the strict logic of the clock. The result is a compact puzzle that rewards close attention to the arrangement of people in a single evening, and a demonstration of the method that would define the detective's later career. It marks an early outing for a partnership built on contrast: instinct against deduction, action against thought.