A dense London fog finds a consulting detective restless until his elder brother's unexpected visit focuses attention on the puzzling death of a young government clerk found on the underground with no ticket and a crushed skull. The inquiry reconstructs the clerk's last hours, follows physical evidence from the railways to domestic scenes, and exposes the disappearance of sensitive documents that point to espionage rather than accident. Careful observation, logical deduction, and coordination with government contacts reveal how the death was engineered and culminate in the recovery of the plans and the unmasking of those responsible.