Amateur detective Albert Campion awakens in a hospital bed with acute amnesia, only to find himself the prime suspect in the assault of a police officer. Campion must navigate a fog of lost memory while attempting to complete a high-stakes government mission that he cannot fully recall. He faces the dual pressure of evading law enforcement and protecting the security of England from an unknown threat. The narrative follows his fragmented perspective as he attempts to reconstruct his identity and purpose while interacting with associates he no longer recognizes.

Readers and critics frequently note the effectiveness of the amnesia trope in heightening the tension of this specific mystery. Discussion often centers on Allingham's ability to balance the personal vulnerability of Campion with the broader political stakes of the plot. Many observers highlight the atmospheric quality of the writing, which captures a sense of urgency and disorientation. The work is often cited for its structural complexity and the way it challenges the reader to piece together the narrative alongside the protagonist. It remains a frequently discussed entry in the series due to its departure from the standard detective procedural format.