The Shadow on the Glass — Agatha Christie

At Mr. and Mrs. Unkerton's party in Greenway's House, Mr. Satterthwaite learns of a haunted window: no matter how many times it is replaced, it always contains the image of a gentleman in a plumed hat. When gunshots are heard, Satterthwaite finds that two of the guests have been shot dead, which is shortly followed by a sighting of the gentleman in the newly-replaced windowpane. Can Mr. Quin shed light on the mystery?