This is a very English country house thriller as people gather at Sir Richard's mansion on a pea-souper of a foggy night. Sir Richard needs to safeguard his reputation by ensuring that the police catch a blackmailer known as The Shadow. Famous people have committed suicide because of the blackmailer. Then a man called Elliot gets killed when he tries to confront The Shadow and is left with a clenched fist design curio in his own clenched fist. This is thought to be a vital clue in finding the identity of the blackmailer.
There are some fairly interesting characters. There are the 'Silvertons' posing as brother and sister. We don't really know their motives clearly for engineering a stay at Sir Richard's house through the excuse of being caught on such a foggy night. There is a dotty aunt in the person of Mrs Boscomb who has some funny lines to say. But the fooling Reggie might be a challenge for you to take as a character.
Suspense grows after we realize The Shadow is in Sir Richard's house and the killings look set to continue. This mystery should keep you guessing. I admit I did actually guess The Shadow's identity shortly before the reveal at the end after an hour of wondering. But I still reckon it's a fairly-well disguised whodunit for fans of the genre.