During the London Blackout, a series of precise assassinations reveals that the killers possess an unnatural ability to navigate and strike in total darkness. Gordon Craigie, the director of the elite Department Z, must identify the source of this tactical advantage before it is weaponized by enemy forces. The narrative follows a high-stakes investigation as Craigie navigates the physical constraints of a city under siege and the logical constraints of a rapidly escalating intelligence crisis. The story is presented through a third-person perspective that emphasizes the procedural elements of counter-espionage and the atmospheric tension of wartime London.

Readers frequently highlight the efficiency of the prose and the classic structure of the mystery. Discussion often centers on the effectiveness of the wartime setting in heightening the sense of vulnerability and urgency. Critics note that the narrative prioritizes the mechanics of the investigation and the professional competence of the protagonist over complex character interiority. The book is often cited as a representative example of mid-century British thriller writing that balances suspense with a focus on institutional response to external threats.