Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Writer: Robert Bloch
Dir.: Joseph Pevney

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols

Jaris: Charles Macaulay
Hengist: John Fielder
Morla: Charles Dierkop
Sybo: Pilar Seurat
Tark: Joseph Bernard
Yeoman Tankris: Judy McConnell
Lt. Karen Tracey: Virginia Aldridge

Stardate: 3614.9

Argelius Two. Scotty has suffered from an accidental head injury caused by a female crew member and now spends some time on Argelius Two to recover. Soon he finds himself in the situation of being a suspect. A young woman was murdered while she was in Scott's company and he's unable to tell what happened. The regent's wife, who has extrasensory perception, tries to use her powers to find out the thruth and is also murdered - Scotty was sitting next her. So is the lie-detector operator. The administrator of the City Chief Hengist is sure Scotty's head injury was more severe than claimed and wants him to be locked up to prevent more murders. 

To add to the difficulty of the situation Scotty cannot remember about his whereabouts when the crimes happened. He definitely was close, because he remembers "something cold, like right out of the slaughterhouse". This something is found to be the murderer – a very old life form which enjoys fear, lives on fear and had previously been on earth known as Jack the Ripper. In Hengis' body it had had plenty of access and chance to murder undetected. It is supposed to have moved along with people traveling from earth. And now it is on board of the Enterprise. In the system, in the engines, in the ventilation system ... 

Since it feeds on fear, the crew finds out that they can fight the alien being by eliminating fear. Dr. McCoy injects everybody (except Spock and Kirk) a substance which makes them totally careless. When the creature tries to frighten people – laughter or total ignorance is the reaction. Brought down, it is finally beamed far away into the open space where it cannot exist and dies.