I like this one too. I'm bummed that Sybo gets killed. I liked her."Wolf in the Fold" by Robert Bloch
We open with Kirk, McCoy, and Scotty enjoying a belly dancer's performance. Scotty leaves with the dancer. When Kirk and McCoy leave, they hear screams and find the dancer dead, with Scotty nearby with a knife in his hand. He claims to remember nothing. Kirk and McCoy discuss an accident he had earlier that gave him a blow to the head (apparently leading to resentment towards all women since a woman caused it) as a possible explanation of his amnesia.
Scotty is interrogated by Mr. Hengist, head of Argelius's police operations. Jaris, the Prefect of the planet, appears with his wife, Sybo, who can use Argelian empathic contact to determine the truth. While she prepares, Lt. Tracy beams down with a "psycho-tricorder" to check Scotty's memories (where was this thing during all the other mystery episodes?) and she too is found murdered, with only Scotty nearby.
Sybo does the ritual, with everyone holding hands like a séance. Sybo says, "Yes, there is something here. Something terrible. I feel its presence. Fear, anger, hatred. Anger feeds the flame. Oh! Oh! There is evil here. Monstrous, terrible evil. Consuming hunger. Hatred of all that lives. Hatred of women. A hunger that never dies. It is strong, overpowering. An ancient terror. It has a name. Beratis, Kesla, Redjac! Devouring all life, all light. A hunger that will never die! Redjac! Redjac!" The lights go out, Sybo screams, and she's dead in Scotty's arms with the knife in her back.
Everyone left (including the dancer's father and fiance) goes to the Enterprise, where Kirk uses the computer's lie detector function. Then they plan to try the psycho-tricorder again. Kirk and Spock go back to what Sybo said and start searching the computer. "Redjac" pulls up "Red Jack", a name given to Jack the Ripper. They speculate that Jack was actually a non-human being that feeds on terror. Hengist gets more and more agitated. Eventually, they start tracing murders of women that are unsolved and find the most recent case was on Rigel IV, where Hengist is from. Hengist tries to flee but falls to the ground dead. Jack is now in the ship's computer.
(Sidenote: Can we just ignore this line? SPOCK: "And I suspect preys on women because women are more easily and more deeply terrified, generating more sheer horror than the male of the species." Oy.)
Jack starts threatening the crew to generate fear, but McCoy gives everyone a sedative. Spock drives Jack out of the computers by having the system compute the value of pi. Jack takes over Jaris, but Spock knocks him out with a neck pinch. Then Hengist gets up and gets the knife. He's subdued and tranquilized and Kirk and Spock take his body to the transporter room where they disperse him into deep space.
I remember being DEEPLY creeped out as a kid by the Jack voice coming out of the computer, and it's still pretty unsettling. I jumped when Hengist's dead body gets up. If one ignores the sexism and the overly jocular ending, it's a decent episode with some truly horrific moments. I'm amused by the idea of the entire Enterprise crew, except for Kirk and Spock, being basically stoned out of their minds for 5-6 hours.
Nit pick about the Deadly Years: the way their hair changes color so quickly. If you were suddenly aged so that your hair was gray, it wouldn't just suddenly be gray, it would have to grow gray. And if you were suddenly cured, your hair wouldn't just suddenly be your original color, it would have to grow back its normal color.