Scotty, Kirk, and McCoy beam down to Orgellius II for therapeutic shore leave. There, they enjoy Orgellian hospitality and belly-dancing. Scotty becomes enthralled with the belly-dancer Kara and invites her for a romantic walk in the fog. Shortly thereafter, Kara is found stabbed to death, with Scott in shock nearby holding the murder weapon. His fingerprints are the only ones on the murder weapon, and he has both a concussion and amnesia.
The chief administrator of Argellius II, the strategically important and only spaceport in the quadrant, Mr.Hengist (from Rigel 4) is put in charge of the investigation. Prefect Jaris suggests that his wife Sybo use Orgellian empathic contact to learn the truth, but Kirk suggests that they beam down psychotricorder to probe Scott's mind while Sybo meditates. Lt. Karen Tracy beams down to administer the test to Scott, but is then stabbed to death. Again, Scott is the only one present and, again, he does not remember a thing. The only other entrance is a door which has been locked for years. Kirk questions two men who were present at the céfe, her father and former fiance Morla (from Contaba Street).
Sybo then holds a séance in which she senses a monstrous evil ancient terror, devouring all light and life, bearing hatred towards women and all living things. She gives the evil the names Boradis, Keslack, Rejick, but the lights suddenly dim. When they come back on, Scott is standing behind her and she has been stabbed. This time, however, Scott does not black out.
Hengist, Jaris, Morla, Kara's father, and the landing party then beam aboard the Enterprise to have Scott analyzed by the computers. The computer verifies that Scott is telling the truth when he claims that he felt the presence of a foul, cold, evil creature. Morla then takes the stand, and the computer verifies that he is not responsible for the murders. Kirk then decides to run the names spoken by Sybo through the computer. It identifies ``Red Jack'' (Rejick) as a name for the mass murderer of 1920's Earth, known more commonly as Jack the Ripper. Spock then speculates that a creature who derives nourishment from the emotion of fear is responsible for the murders. The computer verifies that such a creature could exist, and if so would likely be formless and could exist as electromagnetic impulses. It cites creatures of Alpha Carina 5 who subsist on the emotion of love. The computer also suggests that the creature could take solid form at will, as the Miletus cloud creature of Alpha Madoris can change from gaseous to solid.
A search of crime records shows the following pattern of unsolved mass murders: 1932 Shanghai (7 killed), 1974 Kiev (5 killed), 2105 Martian colony (8 killed), and 2156 Heliopolis on Alpha Eridani 2 (10 killed). Spock notes that all these lie sequentially between Argellius and Earth. The computer then identified Kesla as the name given to an unknown mass murderer of women on Deneb 2, and Boradis as the name given to a mass murderer of Rigel 4 (1 solar year ago).
This immediately implicates Hengist, who recently arrived from Rigel 4. Computer analysis of the knife shows the blade to be made out of boridium, the handle of irinite, and identifies the inlay as originating from the hill people of the Argus river region of Rigel 4. Mr. Hengist makes a dash to get away, but is knocked out by Kirk. McCoy pronounces Hengist dead, but it turns out that has abandoned his body and taken up residence in the Enterprise's computer.
In order to avoid making the creature stronger by allowing him to feed of the fear of the crew, McCoy injects everyone with a tranquillizer. Sulu's reaction is typical, and he remarks that with a armful of McCoy's ``stuff,'' he wouldn't even be afraid of a supernova. The creature is purged from the computer and flees to a new body when Spock commands the computer to calculate pi to the last digit (an impossible task since pi is a transcendental number).
Only McCoy, Jaris, Spock, and Kirk remain to be tranquillized, so Kirk surmises that the creature has taken up residence in one of them. McCoy agrees to inject himself, but when Kirk attempts to inject Jaris, he is met with the outburst ``no, no, kill you all, die, make you suffer.'' Thus revealed, the creature returns to Hengist's body. However, Hengist is quickly subdued and tranquillized, and Spock transports him into empty space with widest possible separation. Kirk and Spock must then wait 6 hours for the tranquillizer to wear off their extremely ``happy'' crew.