At the fashionable French resort of Aix-les-Bains, a murder is discovered: wealthy but naïve Mme Dauvray is found strangled in the salon of her villa, apparently for her famously expensive jewels. Her young English companion, Celia Harland, is missing, and her maid is bound and chloroformed upstairs. A young English inventor who has been courting Miss Harland adamantly protests her innocence, and begs the famous Sûreté detective Inspector Hanaud to investigate …