The Affair of the Bloodstained Egg Cosy

The 1930s country house party packed with crooks, cops, and the usual suspects frame the perfect scene of the crime. This one assembles all the right ingredients--the theft of the fabled diamond necklace, the purloined antique pistols, the secret passage, the ravishing baroness with a past, the body in the lake.... What a pretty puzzle! ""Don't expect me to solve anything,"" Inspector Wilkens announces. But of course, he does: theft, espionage, impersonations (triple, at the very least), blackmail, murder.
The suspects include a noble host and hostess, a jewel thief and foreign agents in disguise, bright young things, mysterious mature men, a Texas millionaire, and, of course, the butler.
Crime connoisseurs will recognize the ingredients for a zany pastiche, a delightful romp paying tribute to the past while titillating the present.
And what does the blood-stained egg cosy have to do with anything?