Poison Unknown

When the death of a well known physician during a dinner party is determined to have been due to some mysterious unknown poison, the natural assumption was that the host of the party had been the intended victim. After all, the doctor had moved to the seat of his host just before he died. And the host had complained to the chief of police himself about a series of anonymous and threatening notes he had received, so much so, that two policemen had been posted outside the house during the dinner. But why would either man have been a target? And how had the untraceable poison been delivered under the eyes of both the guests and the police? Unable to find answers to these questions the police chief turns to his old friend, Professor Harley Manners for help. But can even Dr. Manners, a specialist in abnormal psychology, discover the secrets behind the . . . Poison Unknown!