The Shadow on the Glass

When invited to Mr. and Mrs. Unkerton's party in Greenway's House, Mr. Satterthwaite learns of a haunted window, which no matter how many times it is replaced always contains the image of a cavalier 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 cavalier in the newly-replaced window pane. Can Mr. Quin shed light on the mystery?