A brutal murderer is lose on the streets of London, a number of young women have been found dead. Their corpses have been mutilated and their forefingers have been severed and are missing. Sir George Fenwick (Paul Cavanagh) a prominent member of English society fears he is losing his mind, he has been suffering from black outs and waking in strange locations. Of course there is only one solution, call in Sherlock Holmes (Basil Rathbone) and Dr. Watson (Nigel Bruce). The trail leads Holmes to Lydia Marlowe (Hillary Brooke), a mysterious seductive hypnotist. But could his arch nemesis, Professor Moriarty (Henry Daniell) be the real mastermind. Can Holmes find his way through this web of murder, blackmail, drugs and hypnosis?