After the apparently natural death of wealthy widow Madame Harlowe, a blackmailer accuses her niece of murder by poison—an accusation he knows to be false. The famous French detective Inspector Hanaud is called in, with Madame Harlowe’s death only an excuse to investigate a series of anonymous blackmailings the local police have been helpless to solve.

The police exhume Madame Harlowe’s body and find no trace of poison, defeating the blackmailer—yet there’s a treatise on untraceable poison in the house library. Perhaps the accusation was really true?

A. E. W. Mason had learned about untraceable poisons during his time as a Secret Service agent in 1915.