At Mr. and Mrs. Unkerton's party at Greenways House, Mr. Satterthwaite learns that the house has a haunted window; no matter how many times it is replaced, the image of a gentleman in a plumed hat always seems to come back. He looks very much like a Cavalier soldier, a supporter of the King, from the English Civil War of the 1640s.
Then gunshots are heard, and Satterthwaite finds that two of the guests have been shot dead - soon followed by a sighting of the gentleman in the once-again newly replaced windowpane. Can Mr. Quin shed light on the mystery?
Librarian's note #1: the short story was first published in Grand Magazine in 1924 and then in the print anthology The Mysterious Mr. Quin which was first published in 1930.
Librarian's note #2: the print anthology contains 12 Mr. Quin short stories. Besides those, there are two more found in other collections: The Love Detectives, and The Harlequin Tea Set. Mr. Satterthwaite also appears in Three Act Tragedy and Dead Man's Mirror. All by Agatha Christie, of course!
Librarian's note #3: the entries for all fourteen Mr. Quin short stories can be found on GR by searching for: a Harley Quin short story. Or for Harley Quin. Details such as characters and settings are included for each.