When Watson reads from the newspaper there have been two similar murders near Whitechapel in a few days, Sherlock Holmes' sharp deductive is immediately stimulated to start its merciless method of elimination after observation of every apparently meaningless detail. He guesses right the victims must be street whores, and doesn't need long to work his way trough a pawn shop, an aristocratic family's stately home, a hospital and of course the potential suspects and (even unknowing) witnesses who are the cast of the gradually unraveled story of the murderer and his motive.

In 1840s London, Dr. Thomas Bolton [Boris Karloff] dares to dream the unthinkable: experimenting with anesthesia gases to make surgery pain-free. While doing so, his demonstration before a panel of his peers ends in a horrific mishap with his patient awakening under the knife; he is forced to leave his position in disgrace. To complicate matters, he becomes addicted to his own chemical experiments and gets involved with a criminal gang led by Black Ben [Francis de Wolff], and a ruthless killer Resurrection Joe [Christopher Lee]. Unfortunately, this shady partnership leads Dr. Bolton to further ruin, culminating in his unwitting participation in murder — for which he becomes the first victim of a blackmail scheme.