Superintendent Roderick Alleyn finds his family vacation in the south of France interrupted by a sinister criminal conspiracy involving occult practices. While attempting to balance his undercover duties for the French Surete with time spent with his wife, Agatha Troy, and their son, Alleyn is drawn into a dangerous investigation when a relative of Troy’s becomes entangled with a group involved in drug trafficking and ritualistic activities. The narrative follows Alleyn as he navigates the unfamiliar social landscape of the French coast, relying on his keen observational skills to dismantle a web of deception that threatens his family and the vulnerable women caught in the cult's influence. The story unfolds through a traditional third-person perspective, emphasizing the methodical gathering of clues and the psychological tension inherent in undercover police work.

This novel is widely regarded as a quintessential example of Ngaio Marsh's ability to blend traditional detective tropes with more exotic, atmospheric settings. Readers and critics often note the shift in tone from her earlier, more contained country-house mysteries to the darker, more unsettling subject matter of Satanism and organized crime. The inclusion of Alleyn's family adds a layer of personal stakes that distinguishes this entry from other procedural works of the era. The prose remains sharp and character-driven, maintaining the high standards associated with the 'Queens of Crime' during the mid-twentieth century. It is frequently cited by enthusiasts of the genre as a compelling look at how the classic detective archetype adapts to changing social anxieties.