Four women are murdered and, curiously, all four have been left without their right forefinger. Scotland Yard concludes that a madman is to blame, but Sherlock Holmes suspects otherwise and soon deduces a criminal mastermind is at work.

The illustrious sleuth has few clues to lead him towards another solved case, but he soon discovers a web of blackmail and hypnotism unlike anything he has ever seen. Holmes fears that only one man, his archenemy, Professor Moriarty (Henry Daniell) could be involved in such an evil scheme.