On Christmas Eve, Sir Eustace Vernon hosts a dinner party, though he seems out of spirits and withdraws, promising to join them later in the evening. He does not appear and, in succession, a maid is frightened by a mysterious, knife-wielding figure, Sir Eustace's butler is found dead - and discovered to be a woman (I have learned that there is a sub-genre of Golden Age mysteries loosely known as "cross dressing mysteries for the use of this plot device.) Later,the body of Sir Eustace himself is found, and on the body of the gentleman and his butler is found a red foil bonbon with a threatening.
Sir Austin Kemble, Commissioner of Police is called in and is assisted by Anthony Bathhurst, author Flynn's series amateur detective.
The plot opens in the classic manor house style, and ends with a fairly clever plot twist, but the "in-between" is overlong, with spans of conversations, repetitions and not terribly logical or insightful deductions from Bathhurst, one of the post-Holmes series detectives that sprang up in the wake of Conan Doyle. As for the motive and the twist, so much of it is crammed into the last section, with so few clues to keep a reader intrigued that readers may be tempted to skip much of the overlong, repetitive midsection and get to the end.
I was not familiar with Flynn, who authored nearly 60 novels; unfortunately, this book was not really an inducement to seek out others in his series.