Victorian London is gripped by fear as a serial killer slays an apparently random victim on Hampstead Heath every month. 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 and now work as private investigators and are helping Scotland Yard hunt the so-called 'Heath Maniac.' Their search takes them to houses on the heath where certain residents hold secrets a vivisectionist’s laboratory and the bedside of a dying comedian. They must fight for their lives in an ambush on Hampstead Heath before the maniac is finally exposed in a shattering climax.