In 1860, a person known as the "Haymarket Strangler" is convicted of strangling and slashing five young women to death. Twenty years after the execution in London’s Newgate Prison, James Rankin, a criminologist, suspects that the authorities executed the wrong person. While trying to solve the mystery, he discovers the missing murder weapon, a scalpel, in the convicted man's coffin. Holding the instrument causes him to have a fit and he murders a music hall singer. He has no memory of the crime the next day.
His wife reveals to him that he is the "Haymarket Strangler" of twenty years ago and that she helped him escape from a mental institution when he experienced amnesia. Holding the scalpel again, he transforms into his Hyde-like self and kills his wife. He escapes from the asylum after killing a guard, and is killed by the police while trying to replace the scalpel in the coffin of the "Strangler."