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A series of horrific murders have been committed. Deciding who will be the next victim, the criminal turns... to the alphabet. The unsurpassed detective Hercule Poirot comes into play. Before each murder, an unknown person sends a letter to the detective, telling him where and when blood will be shed again, but the police and Poirot fail to save the victim every time. The chain of evidence leads to the potential killer, an inconspicuous stocking salesman, Alexander Cast. But suddenly the alphabetical scheme fails, and Poirot realises that the killer is someone else...