In London, he is asked by the young and beautiful American Joyce Whipple to keep an eye on her friend Diana Tasborough who owns the estate at Chateau Suvlac.

A large basket is found in a nearby river containing the naked body of Evelyn Devenish; her right hand has been hacked off.

Ricardo's old friend Inspector Hanuad of the Paris Sûreté investigates, working with the local examining magistrate Arthur Tidon.

In the upstairs room that had been lit at 2am he finds a conference table that has recently been re-covered and a large cupboard, opening in the manner of an altar screen, that has been freshly painted white.

Safely back, she explains that during her time at the Chateau Suvlac, she had come to realise that both Evelyn and Diana were besotted with the charismatic Robin Webster.

When Joyce discovered that Robin – once a Catholic priest – had become a renegade and was now the leader of a devil-worshipping sect, she sought to expose him.

Then, Joyce continues, she had slipped a sleeping draft into Diana's drink to ensure she would not be able to attend the rite, stole the masked costume she was intending to wear, and covertly took her place.

[5] Roger Lancelyn Green considered the novel to be one of the best of Mason's detective stories, with only "the rather outré element of devil worship" causing it to be relegated to second place.

[7] In their A Catalogue of Crime (1989) Jacques Barzun and Wendell Hertig Taylor called the novel "the most pretentious and cluttered" of the Hanaud series.