A terse anonymous telephone call reports the murder of a senior government figure, launching a methodical investigation that moves between London and a country estate. The plot follows the gradual unraveling of a misplaced letter, the questioning of enigmatic visitors, and the scrutiny of odd clues such as a telling portrait and a strange apparition, as investigators contend with false leads and a late-night crisis. Nocturnal incidents and a tense car pursuit heighten suspense before the culprit and motive are revealed. The narrative highlights deception, the fallibility of evidence, and the intellectual satisfaction of assembling a concealed solution.