He accuses Betty of poisoning Mrs Harlowe, and junior partner Jim Frobisher is dispatched to Dijon to provide legal advice.

Waberski claims that Betty had bought poison from a shady herbalist, Jean Cladel, but is unable to make good his murder accusation.

But Inspector Hanaud is suspicious when on searching the house he finds in Ann's room a monograph describing Strophanthus Hispidus, a plant from which an undetectable arrow poison can be extracted.

The book includes a plate of an actual arrow smeared with poison and an author's note thanking Simon Harlowe for lending it for research purposes.

When Ann receives an anonymous letter bidding her attend a ball where she will learn the truth about Mrs Harlowe's death, Betty encourages her to go.

He had suspected Betty for some time, he later tells Jim, but unable to prove her guilt had allowed the kidnapping to go ahead to secure hard evidence.

Betty took advantage of the mistake, and before the room was unsealed she moved the clock so that it stood in direct line of sight opposite the door.

"[2] Mason had learned of the properties of Strophanthus Hispidus from W. E. Dixon, a professor of medicine with whom he had served as a Secret Service agent in the Mediterranean in 1915.

They praised the book for its romance, melodrama, good characterisation and humour, but suggested that these things could not compensate for Hanaud's failure to play fair with his Watson [Frobisher].