After arriving at Dytallix B, Data mentions that the Enterprise sensors had spotted Capt. Walker Keel's starship - the USS Horatio, and identified it as an Ambassador-class heavy cruiser". The Enterprise-C, which would appear later on in Star Trek: The Next Generation: Yesterday's Enterprise (1990), is also an Ambassador-class starship.
A star chart featured in this episode, on the wall behind Remmick's chair, was created by the art department and shows several dozen planets and star systems mentioned in Star Trek (1966) and Star Trek (1973). The star chart was re-used in many more TNG episodes and later appeared in the pilot episode of The Sarah Jane Adventures (2007) in the eponymous main character's attic.
When Data is reviewing the Starfleet communications, one of the images that flashes on the screen is a drawing of a bird with a Starfleet uniform, human head and the title "The Great Bird Of The Galaxy". This is a reference to Star Trek creator Gene Roddenberry, who was given that nickname by associate producer Robert Justman. The bird's human head is Gene's and the image is based on a full colour painting that was presented to Gene in 1987 for his 66th birthday.
When Data is commenting on the orders he has just read, the computer interrupts him by saying "Thank you, sir. I comprehend." This is the only time in all of Star Trek when a Federation computer speaks in the first-person narrative ("I"). The non-canon reference work Star Trek: The Next Generation Officer's Manual explains this by stating that the Enterprise-D computer was one of the most advanced ever constructed and was in fact self-aware. It is also one of the only times the computer has expressed "frustration" with its user, more often the frustration is the other way around.
Gene Roddenberry originated the idea for the episode in a single-sentence proposal entitled "The Assassins". Robert Sabaroff expanded this idea to thirty pages, but his version was seen as too expensive. Tracy Tormé was then given the job of rewriting it, but some producers thought the new version was too dark until Roddenberry saw it and endorsed the new version.
This is the last appearance of the Starfleet Admiral's uniform which was seen through the first season of The Next Generation. The uniform, notable for its "triangle pip" insignia, was replaced in season two by an interim uniform which used the more familiar "boxed pip" insignia. By season three, the admiral's uniform was changed again to become the standard which was used for the rest of the series.
The trivia items below may give away important plot points.
The violent ending, with Dexter Remmick's death was ordered by Gene Roddenberry in protest of being bothered with notes from the head office. (Source: Mission Log (podcast) interview with Richard Arnold posted 10 March 2014, story told about 35 minutes into interview)
The original version of this episode made no mention of alien parasites; the conspiracy in question was a military coup within Starfleet. Gene Roddenberry bitterly opposed this, saying that Starfleet would never do such a thing, and the alien angle was introduced. Ironically, "mad admiral" stories soon became a common recurrence on this and other Trek series.
According to "Captains' Logs: The Unauthorized Complete Trek Voyages", the parasites were originally intended to be working for the Borg. The destruction of the colonies in the following episode (Star Trek: The Next Generation: The Neutral Zone (1988)) would then have led into the Borg's introduction in the second season premiere. However, due to the writers' strike of 1988, the Borg storyline was pushed back several months, and the "Conspiracy" subplot was abandoned, also partly due to budget shortages. There was no further on-screen connection or reference to the conspiracy until its mention in passing in Star Trek: The Next Generation: The Drumhead (1991) in the fourth season.
Although the parasites never appeared again on screen in any Star Trek series, they reappear in the Star Trek: Deep Space Nine (1993) relaunch novels, where they are revealed to be mutated Trill symbiotes.
The Vulcan nerve pinch is used in this episode, however unlike Star Trek (1966), a person doesn't fall unconscious after the pinch but grimaces in pain. It's possible, however, that the parasite controlling Savar may not have been performing the neck pinch properly.
Although done in a science fiction manner, much of the story of this episode resembles a real life phenomenon called Capgras Delusion, a disorder in which a person believes a friend, spouse, parent, other family member, or even a pet to have been "replaced" by an identical imposter (sometimes referred to as a doppelganger, a concept appearing in many myths and traditions throughout human history). Patients with the disorder sometimes believe the "doppelganger" is being controlled or is simply not the person they knew, despite all evidence to the contrary. In this episode, the explanation is parasitical aliens and Picard's fears turn out to be true; in real life the explanation is a neurological disorder seen in patients with paranoid schizophrenia, dementia, or brain injury.
The part where the parasite enters Remmick's throat reportedly took many takes because the bulging effect was made by Michael Westmore blowing into air bladders under a false neck, and Cliff Bole kept trying to make Westmore hyperventilate from the exertion.
the alien in this episode holds a striking resemblance to the g'aould parasite which plays a dominant role in the Stargate franchise. From its method of control, strength, and how it attaches to the host. Both also leave a tell-tale mark on the neck.
A mold of Paul Newmans face was filled with raw meat and then blown up to create the effect used when Picard and Riker fire on Remmick, but both Rick Berman and Peter Lauritson were concerned that it was too graphic. Dan Curry invited his six-year-old son to watch the episode in order to test how children would react to it; the boy reportedly liked it so much that he suggested the creation of a Remmick action figure whose head would blow up by pressing a button. This resulted in Berman deciding to air the episode uncut with the full sequence included.
Anatomy goof: The good doctor shows a scan while explaining how the parasite latches onto the brain stem and/or spinal cord but then states the fantastic strength comes from the parasite stimulating the adrenal glands. Those are nowhere near the brain. They are located in top of kidneys, below the heart, lungs, and diaphragm. Maybe Cmdr Remick got strength from adrenal stimulation, since the alien queen came out of the abdomen after his head blown off. That would be pretty close to the adrenal glands!