London, 1888. After Dr. Jack York's misdiagnosis ends up costing a patient's life, he has descended into absinthe addiction, and spends his evenings at a brothel. Although tempted, he ignores the women and only reads his book. On his way home one evening, he hears two women inside a warehouse. When he enters he witnesses a being of blue light ripping out of the woman's chest, and entering the other woman...
Soon Jack discovers an alien creature that lives inside women and kills them: after a time it leaves its host body by bursting out of the chest and enters another human host which it has selected.
Jack follows this creature, trying to unveil the truth. His only lead is a green substance that the infected women cough up. But in the meantime the bodies are discovered and police are searching for a mass murderer they dubbed "The Ripper". The creature uses this to its advantage, planting further evidence that Jack is the ripper.
In the end Jack is discovered killing the alien, while still inside the body of a woman, and is because of that sentenced to a mental institute. His fiance Ellen visits him to tell him that she is leaving for America. Directly after the Inspector visits Jack to tell him that he's retiring and is going to follow Ellen to America. The Inspector calls America "the land of opportunity" and coughs up the same green substance as the infected women.
He leaves Jack alone in the asylum with the message "...Don't worry, she'll hardly feel a thing..."