Hengist
John Fielder

Jaris
Charles MacCauley

Sybo
Pilar Seurat

Tark
Joseph Bernard

Morla
Charles Dierkop

Yeoman Tankris
Judy McConnell

Lt. Karen Tracey
Virginia Alderidge

Kara
Tania Lemani

During shore leave on the planet Argelius II, Kirk, Scotty and McCoy are watching an exotic belly dancer called Kara. When she finishes her performance she comes over and invites Scotty, who is quite taken with her, for a walk in the fog outside which he happily agrees to.
This comes as a relief to McCoy who has been worried about Scotty's attitude towards women following his involvement in a recent accident caused by one.

Kirk and McCoy decide to move on but hear a scream as they are leaving. They rush out into the fog where they discover Scotty, standing over Kara's dead body, holding a knife. The subsequent investigation, which is headed by the city Administrator Mr. Hengist from Rigel IV, turns up no other evidence and Scotty maintains that he has no memory of the incident. Prefect Jaris and his wife Sybo agree to use Argelian empathic contact on Scotty to get to the truth and the scene moves to the Jaris house where all the 'witnesses' have been assembled.

Lieutenant Tracy is beamed down from the Enterprise with a psycho-tricorder to make a regressive memory check on Scotty. They are left alone in a room for the examination but, on hearing her scream, everybody rushes in to find Tracy stabbed and Scotty lying unconscious. On recovering, Scotty is distraught to learn of the attack as he cannot remember anything about what happened in the room. Hengist arrives at the house with Kara's father, Tark, and her lover, Morla. Sybo prepares herself for the contact and they all take a seat around a table, at the centre of which is a ceremonial flame.

Sybo, now in a trance, becomes aware of a great evil in the room and cries out the names 'Beratis', 'Kesla' and Redjac', following which all light in the room is suddenly extinguished. When light is restored Sybo is discovered to have been stabbed and Scotty is holding her dead body. Kirk manages to persuade Hengist and Jaris to reconvene the investigation aboard the Enterprise where they will have the help of the ship's computer and other advanced facilities to help them.

Scotty is cross-examined and maintains that he has no memory at all of the first two murders. When Sybo was killed he recalls moving toward her in the dark only to be held back by a resistance of some sort. On being asked outright whether he killed her he denies it and the computer's lie-detecting ability confirms that he believes he is telling the truth. Morla is similarly questioned and also declared innocent. Kirk asks Spock to run a computer check on the names that Sybo called out before she died. He comes up with the answer that 'Redjac' or 'Red Jack' is an old Earth nickname for Jack The Ripper.

Further checks reveal a string of similar unsolved murders on different planets in a direct line between Earth and Argelius over a period of time. Spock surmises the possibility that a long-lived incorporeal alien life form that feeds on fear may be responsible. The computer further reveals that several recent murders on Rigel IV were carried out by a murderer nicknamed 'Beratis'. The hearing continues and Hengist, who is from Rigel IV, is asked to take the stand. The computer announces that the current murder weapon originates from Rigel IV, at which point Hengist attempts to escape and, when Kirk prevents it, dies for no obvious reason.

The alien entity, which had been inhabiting and controlling his body, enters the ship's computer and starts to take control of the Enterprise. McCoy is ordered to sedate the crew to reduce the fear that the creature seems to live on. By tying up the computer functions in calculating the absolute value of pi, Spock forces the entity out. It first moves into Jaris, who is subdued by a Vulcan neck pinch, and then back into Hengist's body that eerily comes back to life. He is overpowered and sedated, after which the body and the alien are beamed into space on a wide dispersal transporter setting.