After Scotty is granted therapeutic leave after a head injury, Kirk and Bones take him to Argelius II to visit a nightclub that has a sort of gypsy/Arabian Nights theme going on. Scotty seems to be recovering nicely and chats up a dancing girl named Kara. Kirk and Bones decide to leave Scotty alone with his new friend, but stop when they hear a screech from an alley. Kara is dead and Scotty is standing over her as if in a trance.
An inquest is held. Administer Hengist calls for Scotty's immediate arrest. Scotty can't remember a thing. When Yeoman Tarkis, beamed down from the Enterprise with a psychotricorder to recover Scotty's memory, turns up dead and Scotty unconscious nearby, things look bad.
Jaris, the Argelian leader, introduces them to his empathic wife, Sybo, who will use her Psychic Powers to deduce who the killer is. Spock notes that although psychic powers have been confirmed to exist in some Argelians, it's not the kind of thing that would meet the standard of evidence in a Federation courtnote and suggests they take the matter out of the Argelians' hands. Kirk, however, states that they're obliged to let the local authorities proceed in their own way.
Sybo goes into her trance and states she senses a great feeling of anger and hatred towards women and a desire to feed on fear. She mentions the names "Kesla, Boratis, Redjac" and states that he has a hunger that will never die. But Sybo will. When the lights come back on, Sybo has a knife in her back and Scotty has blood on his hands.
Scotty, Jaris, Hengist and two other persons of interest (Kara's father Tark and her ex-boyfriend Morla) are beamed aboard the Enterprise with Kirk and Bones. Scotty is further questioned, using the ship's computer as a lie detector. The conclusion is that Scotty does not know if he committed the murders. He remembers black outs, coldness and a stench like a slaughterhouse. In the face of Hengist's insistence that Scotty is obviously guilty, Kirk suggests that they attempt to learn what they might from Sybo's last words. A search of the computer records for the name "Redjac" takes the investigation in a completely unexpected direction.
Tropes for this episode include:
- Absentee Actor:
- And I Must Scream: Redjac's fate. When he's tricked into possessing Hengist, Scotty uses the transporter to send the body into deep space, far from Enterprise. Kirk exposits that when Redjac is forced to leave the dead body, he'll disperse and fade into powerless energy forever without something to concentrate his form.
- As You Know: The episode does not trust Star Trek viewers to know that pi is a nonrepeating decimal.
- Belly Dancer: The first thing we see in the episode is Kara performing at the nightclub with finger cymbals. Tania Lemani is a professional Middle Eastern dancer. Like most Hollywood bellydancing, her performance here is a flourishing improvisation with some bellydancing elements.
- Bloodless Carnage: Kara is stabbed "dozens of times", Lt. Tracy is stabbed "over and over again", yet neither bodies bear any wounds and there's no blood at all until Sybo falls over with a blade in her back.
- Clear My Name: Kirk's initial goal is to absolve Scotty of murders that he knows he couldn't commit.
- Courtroom Episode: Scotty is accused of multiple acts of murder and Captain Kirk effectively acts as his defense attorney.
- A Day in the Limelight: Probably Scotty's most developed part in the entire series.
- Did You Just Punch Out Cthulhu?: Redjac is all but stated to be an Eldritch Abomination who's been killing for millennia...Kirk and the crew get rid of him for good.
- Emotion Eater: Redjac feeds on fear.
- Everybody Laughs Ending: Justified. Redjac feeds on fear, so Bones injected everyone with mood lightening drugs that made everyone super cheerful. Kirk observes that for the next few hours, he's going to have the happiest ship in the fleet, and not a lot of work's gonna be done.
- Halfway Plot Switch: The episode begins with Scotty framed for murder, but at the end of the questioning scene, we've got a much different problem.
- He-Man Woman Hater:
- Scotty was said to have a "total resentment of women" because a woman caused his accident. He seems over it by the time he meets Kara.
- Redjac. It is claimed (by Spock — Leonard Nimoy must have hated saying this line) that since women would be more easily scared, Redjac prefers going after females. Of any species.
- Historical Rap Sheet: Redjac is a noncorporeal lifeform which has been a serial killer on several planets, including Earth where it was Jack the Ripper as well as a few other unnamed killers (in China in 1952 and Kiev in 1974).
- Informed Attribute: We never actually see Scotty demonstrating the total hatred of women that he's claimed to have, though he is mentioned to been on the tail end of healing.
- Instant Sedation: The tranquilizers McCoy injects everyone with have an instant effect.
- Laser-Guided Amnesia: Scotty just can't seem to remember what happened when he was alone with those women.
- Lie Detector: The ship's computer can act as one. We must assume it is more accurate than modern polygraphs.
- Lights Off, Somebody Dies: How Sybo is killed.
- Loafing In Full Uniform: Kirk thought to wear his casual green tunic, but all three of them are taking in the delights of hedonistic Argelius' night spots in full uniform. Apparently Starfleet doesn't have any concerns about how its members represent it off-world. On the plus side, the future evidently has vastly more comfortable uniforms.
- Men Are the Expendable Gender: Subverted, as the three people hacked to bits are all women. Most likely so we know Redjac really is a monster, as he actually kills girls.
- Mouthful of Pi: Spock prevents Redjac from taking over the ship's life support systems by ordering it to calculate the last digit of pi.
- Never the Obvious Suspect: Was it Scotty, who was found standing over the victims holding the murder weapon? Or was it the ghost of Jack the Ripper?
- Non-Standard Prescription: Doctor McCoy had Scotty visit a club full of scantily clad babes featuring a bellydancer saying it was prescription.
- Noodle Incident: Kirk never does get to explain what's so great about his favorite Argelian nightspot. When he mentions it to McCoy, McCoy cuts him off with an enthusiastic "I know the place you mean", and when he tries to interest Spock, he falters and gives up mid-sentence in the face of Spock's disapproving look.
- Not Using the "Z" Word: Redjac is clearly intended to be a demon, but the word "demon" is never used in the episode.
- Ominous Fog: Odd juxtaposition. Scotty describes the fog of Aberdeen to be almost romantic. (He feels that way about anything Scottish.) Kirk and Bones leave the club to be surrounded by a thick fog not dissimilar from what would be found in a film on Jack the Ripper. Which was probably intentional, given that the killer has gone by the name of Jack Ripper at one point...
- People Puppets: How Redjac commits his murders, being a Puppeteer Parasite. Maybe Dead Person Puppet in Hengist's case.
- Perfect Pacifist People: The Argelians have achieved a society devoted to pleasure where violent crime has been unknown for over a century.
- Psychometry: Sybo's gifts are said to include the ability to gain impressions from inanimate objects, although she doesn't get a chance to demonstrate it on the murder weapon because it goes missing and only reappears after her death.
- Red Herring: The sinister-looking guy who leaves just a few minutes before Kara and Scotty. During the investigation it comes out that he's Kara's fiance, and their relationship has been troubled because he got jealous over her friendly interactions with other men in the course of her dancing. But then he's cleared by the computer's lie detector and the murderer turns out to be someone entirely different.
- Similarly, the possibility of Scotty's offscreen head injury (caused by a woman; nearly healed in-episode) causing functional amnesia, regarding his role in the murders. Ten seconds into questioning, the Enterprise computer answers 'Negative' to this being the case, much to surprise in-universe.
- Reverse Cerebus Syndrome: Starts out seriously enough with Scotty being accused of murder, and gets even worse when Redjac takes control of the Enterprise's computer, potentially being able to kill everyone on board. After the heroes force him out of the computer by having it attempt to compute the last digit of pi, though, Redjac is forced back into Hengist, who has already been tranquillized, and becomes a total joke. Thanks to the mass tranquilization, there is an Everybody Laughs Ending.
- Spooky Séance: Sybo hosts one. It's her last.
- Wham Line: In-universe, at the least. Redjac's name is revealed to be a nickname for a mass-murderer of women, but it has a similar name from Earth... Jack the Ripper.