The Enterprise is on its way to Pacifica, a water world (no relation to the Kevin Costner movie of the same name) for some much-needed R&R... a good precaution, considering the crew seems so out of it, they respond to an order for warp 6 with going to full impulse.
However, a priority call to Picard interrupts the mission, as it's Picard's old friend, Captain Walker Keel, on the line. He demands for the ship to detour to an abandoned mining planet for a secret meeting. Picard is confused but curious, and humors him. Upon arriving at the planet and beaming down, Picard is confronted by not just Keel, but two other captains, who try to Bluff the Impostor to make sure Picard is really Picard before telling them of their concerns of a subversion in the Federation. The evidence is both vague and disturbing: mysterious disappearances and deaths in the ranks, bizarre orders and personnel shuffles, among other things. Picard doesn't buy it, but agrees to keep his eyes open for anything unusual that would corroborate their stories, assigning Data to look over Starfleet's recent activities upon returning to the ship as a precaution before heading back on course to Pacifica.
Unfortunately, the conspiracy decided to make its presence known shortly thereafter; the Enterprise comes across the destroyed wreckage of Keel's ship, and through his research, Data is able to confirm many of the odd occurrences Picard was told of, leading everyone to assume the worst: secret invasion. Since anal Skrulls wouldn't be invented for several years, Picard decides to head to Starfleet Headquarters to figure out just what the hell is going on. Once there, Picard and Riker meet with a trio of admirals, all of whom seem to be acting very strangely. Things get weirder when we learn that one of them is Admiral Quinn, who, in a previous episode ("Coming of Age"), started the rumors of the conspiracy, but now insists he was only speaking metaphorically about acquiring new members of the Federation. While Riker is still not convinced, Picard is, and orders him to keep an eye on this admiral while he goes on a tour of the Enterprise.
Once on board, Quinn introduces Riker to a familiar-looking brain slug, batting him around like a rag doll when he refuses to become one with the thing. When Worf and Geordi fail to stop him, Quinn is eventually subdued by Dr. Crusher, and an investigation reveals a similar brain slug within him. Apparently, these are the things that are trying to subvert the Federation, and as Picard soon learns, they've made their way into the highest levels of Starfleet. Fortunately, Riker recovers enough from his beating to pull off a successful fake-out to rescue Picard from assimilation, and together, they face off against the mother alien, possessing Quinn's second-in-command, Commander Dexter Remmick, who defiantly insists they only wanted "peaceful coexistence". The episode ends on a Red Herring Twist, with a homing signal being sent out to the Delta Quadrant, meant to be the prelude to the Borg, but ultimately being left hanging as an The End... Or Is It? cliffhanger, never to be followed up upon.
Except, that is, in the Star Trek Expanded Universe, where the story got several contradictory resolutions. In the Star Trek Novel Verse they're revealed to be mad, genetically engineered Trill symbionts, essentially the Trills' equivalent to Romulans and in Star Trek Online, they're Solanae-engineered infiltrators made for the Iconians to infiltrate the Vaadwaur.
This episode contains examples of:
- Aborted Arc: The series never followed up on the final cliffhanger in this episode—at least not straight: The Borg were originally going to be the follow-up to this episode and be an insect-like race, but that was too expensive.
- Absentee Actor: Wesley again does not appear.
- Alien Lunch: The slug-controlled people eat live maggots.
- Artistic License Biology: Yeah... adrenaline does not work like Popeye's spinach. The stuff is good, but its not being an out of shape old man kicking the asses of three fit young guys without a bruise and then tanking a phaser stun good.
- Then again, the bluegill also mentions taking "vitamins" while whaling on Riker, though that's probably BS.
- Arc Number: Picard gets a message code 47 in the teaser; the number is repeated four times within a three minute period.
- Batman Gambit: The controlled Starfleet officers reveal they knew about the secret meeting (as one of them was there). They orchestrated events so that Picard would head to Earth and be in a position to be overtaken.
- Beware the Nice Ones: Doctor Crusher just happens to have a phaser on hand to deal with Admiral Quinn, who has already taken down Riker, Geordi, and Worf.
- Big Damn Heroes:
- Dr. Crusher comes along just in time.
- It doesn't look like it at the time, but Riker appearing at the dinner.
- Blatant Lies: "You don't understand. We mean you no harm. We seek peaceful co-existence!"
- Body Horror: Good God, the mother-alien hiding underneath Remmick. And before that, we "get" to see Remmick's head explode. This is not your average TNG episode.
- Brainwashed and Crazy: Everyone affected by the slugs.
- Brick Joke: Many times during the series do the organic beings express exasperation when Data is too precise or elaborates too much. It's quite something else when the Enterprise's computer does so while he is expressing wonder at talking to himself.
- Combat Medic: Doctor Crusher, with her prescription of a phaser set to kill.
- Darker and Edgier: As noted above, this is an unusual TNG episode. Besides the graphic violence during the climax, the tone is consistently tense, the threat quite insidious, and Starfleet officers are basically shooting at each other.
- Deadpan Snarker: When Worf asks if he's all right, Geordi replies that if he could see, he'd be seeing stars.
- The End... Or Is It?: The final scene is Data reporting to Picard about the beacon and viewers hearing the signal.
- Fake Defector: Riker fakes being infected by the slugs so he can get into the conference room and help Picard. It fools everybody, Picard included.
- Gorn: The death and destruction of Remmick and the mother parasite inside him was very controversial when it first aired. The BBC cut several minutes of footage when first airing this episode in the United Kingdom, notably this part, and Canada's Space network preceded the episode with a viewer discretion warning, the only episode of The Next Generation to receive one. The episode did win an Emmy Award for Outstanding Achievement in Makeup for a Series, though.
- Hates Baths: Worf's dislike of bathing extends to swimming of all sorts.
- Hope Spot: Picard has one when he's trying to escape from a room of possessed admirals and runs into Riker, only to discover that he's possessed, too. Subverted, though, when Riker reveals that he was only acting possessed so he could get close enough to shoot the admirals.
- Keystone Army: The parasites are unable to survive after the mother parasite is destroyed.
- Literal-Minded: Data, as usual, regarding the experience of swimming in a real ocean vs. one on the holodeck.Troi: Have you ever been for a real moonlight swim?
Data: One can swim in moonlight?
- Make It Look Like an Accident: Shortly after Keel meets with Picard, his ship is found destroyed. When Picard asks, one of the mind-controlled Admirals says it was caused by "an implosion", and blames Keel's incompetence for the accident (after Picard has spent the whole episode talking up Captain Keel as one of Starfleet's best). In addition, several other officers have died off-screen in what were supposed to look like accidents.
- Nice Job Fixing It, Villain!: The possessed Quinn initially came aboard the Enterprise in order to place Dr. Crusher under parasite control. However, he shows it off to Riker on the grounds that it likes him, which leads to the crew finding out what's really going on.
- Orphaned Punchline: After the opening First Officer's Log, we pick up the end of a humorous anecdote Geordi is telling to Data. Data responds by explaining the joke and giving a rather creepy laugh.
- The Power of Friendship: Picard initially has his doubts about the conspiracy theory, but he implicitly trusts Keel—calling him a lifelong friend along with Jack Crusher.Picard: Friendship must dare to risk, Counselor, or it isn't friendship.
- The Power of Trust: Keel and the others warn Picard not to trust anyone, as there's no way to be sure a person is who they say they are. note However, Picard trusts his crew unconditionally.
- Punch Catch: Quinn does this successfully with Riker twice. First Riker throws a punch after Quinn painfully grabs his arm. Then as he picks himself off the ground Riker throws another punch which Quinn catches, knocks aside, and viciously backhands Riker.
- Punch! Punch! Punch! Uh Oh...: Neither Worf nor Riker succeeds in causing any damage to the possessed Quinn.
- The Reveal: Captain Scott was under parasite control the entire time.
- Riddle for the Ages: Whatever happened to the signal sent to a remote area of the galaxy by the parasitic aliens at the end? We'll likely never knownote .
- They did get expanded on in STO.
- Right Hand vs. Left Hand: Data uses this analogy when describing the clandestine orders he discovered.
- Ripped from the Headlines: This episode was meant to be a commentary on the Iran/Contra scandal.
- Sequel Hook: A followup episode was planned, as the end seems to strongly suggest, but elements of it were instead folded into the Borg stories.
- Shout-Out: Someone in the scriptwriting team of the series seems to be a big fan of Albedo: Erma Felna EDF, a furry comic, as two members of Starfleet (whose names appears in the Command Order) are named Itzak Arrat and Toki. In Albedo, Arrat is also a spaceship captain and Toki is the main heroine's best friend (who happens to be female).
- Spotting the Thread: Picard's admiral buddy Quinn tells him and Riker that his mental fatigue was due to "difficulty assimilating other races into the Federation". It's treated as a Wham Line, triggering "Something is dreadfully wrong" music and Picard telling Riker that Quinn is Not Himself. Note that this is before the Borg gave "assimilate" such an ominous connotation.
- Squick: Picard's In-Universe reaction to everyone eating live maggots for dinner. Also his reaction to the very sight of the mother alien possessing Commander Remmick.
- Stock Footage: An establishing shot of Starfleet Command (a combination of miniatures, matte painting and live action extras) is recycled from Star Trek IV: The Voyage Home.
- Story-Breaker Power: Invoked. It's why Picard has Data review six months worth of Starfleet directives. With his ability to go over information at such a fast rate, Data can see questionable patterns that anyone else would've missed. When he is done, he is absolutely certain there is a conspiracy at work.
- Stun Guns: Averted—Crusher tells Picard that stun settings do nothing, and that he has to set his phaser to "kill".
- Actually stun does work on the parasitic hosts, as shown with Quinn being phasered by Crusher, but only after multiple shots, which means it was better to use the kill setting in order to avoid being shot while focusing on one target such as when Picard and Riker was attending dinner at Starfleet Headquarters with multiple compromised people.
- Surprisingly Good Foreign Language: Perhaps Jonathan Farwell (Walker Keel) is not really fluent in French, but his pronounciation of "Jean-Luc" is correct, au contraire to many actors on this show.
- Thinking Out Loud: Data discovers a new human idiosyncrasy.Data: Startling. Quite extraordinary, in fact.
Computer: Directions unclear. Please repeat request.
Data: That was not a request. I was simply...talking to myself. A human idiosyncrasy, triggered by fascination with a particular set of facts. Or sometimes brought about by senility. Or used as a means of weighing information before reaching a conclusion. Or, as a—
Computer: Thank you, sir. I comprehend.
- Tidally Locked Planet: Dytallix B was a world inhabited only by the Dytallix Mining Company. Due to the temperature extremes on either face of the planet, the company placed its facilities in the twilight region.
- Took a Level in Badass: Thanks to the aliens, an old man like Quinn has augmented strength.
- Trust Password: Keel tests Picard about their shared history, as apparently those who are infected by the parasites can't access personal memories. Keel says that other officers he suspected were bluffing their way through talks of old times.
- What Happened to the Mouse?:
- Other than Quinn, it's not made clear if the other people the parasites infected survived.
- We've never found out what happened with the signal Remmick sent.
- The Worf Effect: The possessed Admiral Quinn easily hurls Worf over a table. Even Riker puts up a better fight than he does. Then Dr. Crusher calmly walks in and phasers his ass. Multiple shots at high setting were required, though, one lasting as long as three to four seconds.