Who Killed His Royal Highness?

The Grand Duke, Maximilian Karol Alexandrei Kadarian, collapses at a banquet. The Grand Duke was famous locally as phony royalty, who owned a fabulous emerald necklace, now missing. The suspects include his former wife, his secretary, the owners of the banquet hall (two former vaudevillians), the ghostwriter of his autobiography, a ""reformed"" safecracker, another Russian noble now working as a director, and the mysterious and elusive ""Charlie Prince"".