Lord Edgeware Dies: by Agatha Christie.

There is more than one person who might murder Lord Edgeware, but one has a watertight alibi - the wife, an actress of yore famed for beauty and style, who was at a very well attended dinner - and the other, a young man, was at late night show of film which no one can corroborate. Both had entered the house at the relevant time, and either could be the murderer.

Then there is another actress, a minor one who resembles the Lady Edgeware, found dead. Did she do it, using the name of the better known actress, and if so why?