When a series of suspicious signals and a grisly death occur at a remote New Zealand health resort, Chief Inspector Roderick Alleyn must navigate a web of wartime espionage and local tensions to identify a killer. Tasked with uncovering German agents during the Second World War, Alleyn finds himself in the isolated region of Wai-ata-tapu, where the victim, a man of profound unpleasantness, has been murdered in a boiling mud bath. The narrative, presented in a classic third-person perspective, forces the inspector to sift through a cast of suspects ranging from disgruntled English exiles to the local Maori population. Alleyn must contend with the physical hazards of the geothermal landscape while untangling the complex social dynamics that mask the true culprit's identity.

Ngaio Marsh herself held this novel in high regard, often citing it as her most accomplished work of fiction. Critics frequently highlight the atmospheric New Zealand setting as a standout feature that distinguishes the book from other Golden Age mysteries of the period. Readers appreciate the complex interplay between the wartime spy plot and the traditional whodunit mechanics that Marsh masterfully executes. The consensus among scholars of the genre is that the book represents a high point in the Roderick Alleyn series, showcasing the author's ability to blend social commentary with a tightly constructed puzzle. The work remains a staple for those interested in the evolution of the detective novel during the mid-twentieth century.