A routine afternoon excursion for two cousins is abruptly derailed when they are conscripted into a clandestine intelligence operation to neutralize a national threat. Mark and Mike Errol find their plans for a quiet boat ride in Richmond shattered by their forced recruitment into Department Z, a secretive arm of British Intelligence. They must navigate a high-stakes investigation against the League of the Hundred-and-One, a shadowy organization threatening the stability of Britain. The narrative follows the cousins as they transition from civilian life into the dangerous world of espionage, facing physical peril and logical puzzles designed to destabilize the nation. The story is presented through a third-person perspective that emphasizes the rapid escalation of the conflict.

Readers often note the brisk, efficient pacing that characterizes John Creasey’s work within the Department Z series. Discussion frequently centers on the contrast between the mundane lives of the protagonists and the extreme circumstances of their recruitment. Critics highlight the author's ability to maintain suspense through the constant threat posed by the elusive League of the Hundred-and-One. The narrative is frequently cited for its focus on plot-driven action rather than deep psychological introspection. Many readers appreciate the classic British thriller atmosphere that defines this era of the genre.