Doctor David Garth fell in love with the beautiful widow Betty, Lady Calder. Inspector Twigg of Scotland Yard tries to warn Doctor Garth about Lady Calder’s “colorful” past, but it takes all the persistence of David Abbot Callingford, the assistant to the Director of Scotland Yard, for Garth to understand that among other things Betty danced for three seasons in the Moulin Rouge and, it seems, joined a group of Satanists in Paris. She is also thought to be a blackmailer responsible for at least two suicides.
However, Betty herself suggests she has been wrongly accused of schemes in which her sister Glennis is at fault. Glennis is found dead on the shore near a bathing pavilion. And Betty is suspected of preparing her death (though no one can say how that could have been done). All of Doctor Garth’s knowledge and experience are needed to solve this riddle and punish the culprit.