Sir Charles Hakeworth is garrotted at a New Year's Eve masked ball held at Montague Hall by Lord "Monty" and Lady Felicia Montague. A death threat, stating that the Red Death was coming, had already led to Inspector Mallory closing the estate, trapping the killer. Father Brown with the aid of Mrs McCarthy, Sid, and Bunty look to discover the killer from four people who knew Hakeworth would be present; his wife, his doctor, Bunty's friend Ruth Moulton, and Robert, Earl of Finchmore whose father committed suicide after Hakeworth's adulterous affair with Robert's mother. Father Brown sees a figure in a red mask escaping after stabbing Moulton. Lady Felicia, unhappy with her marriage, has a decision to make when a one-way airline ticket to New York arrives from Flambeau.