Victorian London is gripped by fear as a serial killer slays an apparently random victim on Hampstead Heath every month, each with a single knife thrust. Two men begin to suspect a mysterious link between the victims: Major Henry Steele and ex-Sergeant Mason have been discreetly retired from Military Intelligence following the suspicious death of a dangerous German agent in the Middle East.
Now they work as private investigators, and are helping Scotland Yard hunt the so-called 'Heath Maniac'. Their search takes them into large Heath-side houses where certain residents seem to have secrets, to the offices of shady lawyer, to the laboratory of a vivisectionist, back-stage at a London music-hall, and later at the bedside of a dying comedian. Steele and Mason find themselves fighting for their lives on Hampstead Heath, before the Maniac is finally exposed in a shattering climax.