The Smiling Cadaver — John Newton Chance

Clearly it was to be no ordinary house party at Ramboro Castle. The couple's undercover mission was the investigation of the possible substitution of a bogus Lew Grogan for the genuine article. They discovered that the marriage lines of their fellow guests would not have stood up under cross-examination, and some of the staff at the castle would have made excellent prison warders, others would have been the inmates. Then the diamonds started to appear, and the corpses to pile up...