The novel follows Maxwell Cheyne, a returned naval officer and amateur yachtsman whose quiet Devon life is disrupted by a puzzling encounter in a Plymouth hotel. A stranger's business proposition leads to theft, secret documents, and a trail of clues—ranging from a clay-marked shoe to torn hotel accounts—that unravel into a layered criminal case. The narrative alternates measured descriptions of character and setting with procedural detection as local police and a determined investigator assemble forensic evidence, trace movements by sea and land, and piece together motives and betrayals until the mystery's resolution.