S03E03 - The Affair at the Victory Ball

Chief Inspector Japp asks Poirot to assist Scotland Yard in the strange events which took place at a recent costumed Victory Ball. A group of six people, headed by the young Viscount Cronshaw, attended dressed in the costume of the Commedia dell'arte. Lord Cronshaw was Harlequin, his uncle, the honourable Eustace Beltane, was Punchinello and Mrs. Mallaby, an American widow, was Punchinella. In the roles of Pierrot and Pierette were Mr. and Mrs. Christopher Davidson (he being a stage actor) and finally, Miss 'Coco' Courtney, an actress who was rumoured to be engaged to Lord Cronshaw, was Columbine.
The night went badly from the start when it was obvious to the party that Cronshaw and Coco were not on speaking terms. The latter was crying and asked Chris Davidson to take her home to her flat in Chelsea. When they had gone, a friend of Cronshaw's spotted Harlequin in a box looking down on the ball and called up to him to join them on the main floor. Cronshaw left the box to join them but then disappeared. He was found ten minutes later on the floor of the supper room, stabbed through the heart with a table knife, his body suffering a strange stiffness. To compound the tragedy, Coco Courtney was found dead in her bed with an overdose of cocaine, it being found out afterwards at the inquest that she was addicted to the drug.
Poirot starts to investigate, finding out to everyone's puzzlement that Cronshaw was emphatically opposed to drugs, that Beltane's costume had a hump and a ruffle and that a curtained recess exists in the supper room.