Victorian London is gripped by fear as a serial killer slays an apparently random victim on Hampstead Heath every month. Two men begin suspect a mysterious link between the victims: Major Henry Steele and ex-Sergeant Mason have been discreetly retired from Military Intelligence and now work as private investigators, and are helping Scotland Yard hunt the so-called "Heath Maniac." Their search takes them into Heath-side houses filled with the residents’ secrets, to the offices of shady lawyer, a vivisectionist’s laboratory, and back-stage at a London Music-hall. They must fight for their lives in an ambush on Hampstead Heath, before the maniac is finally exposed in a shattering climax.