Kirk and McCoy decide to take Scotty, who is recovering from a head wound accidentally caused by a female crew member, to a nightclub on the planet Argelius II. Scotty becomes infatuated with a lovely dancer at the club and they leave together. In the meantime, Kirk and McCoy decide to sample some of the planet's other pleasures and leave.
A scream sends them to a foggy alley to find the dancer dead with Scotty holding a bloody knife. McCoy suggests that perhaps Scotty's subconscious distrust of women since his accident has manifested itself in murder.
Hengist, the local authority, wants to arrest Scotty, but Kirk intervenes and seeks the help of a priestess of an old psionic cult. Unfortunately, she's killed and once more the blame seems to fall at Scotty's feet. Before she dies, the priestess says that something with an insatiable hunger and hatred of women is present in the room. Scotty still claims to have amnesia during the time when the women were killed.
In the end, the entity turns out to be an ancient life form, Redjac, previously known on Earth as Jack the Ripper. It now appears in true form: a noncorporeal vampire who thrives on others' fear. It preys on women because they are more easily frightened. It has been living in the body of Hengist and, when discovered, kills Hengist and flees to the U.S.S. Enterprise.
McCoy administers tranquilizers to everyone on board so that the creature cannot evoke fear. Enraged, it is forced to return to Hengist's body. Kirk, knowing this would happen, beams it into space at maximum dispersal, where it will die for lack of nourishment.