On the bridge, Worf is field testing some newly-upgraded torpedoes in an asteroid field. When one of them veers off course (the result of some "improvements" made to the guidance system by Worf himself), a bored Picard decides to take a shuttle out himself to retrieve it. Data joins as well, after leaving Spot in the care of Barclay.
Not long after they leave, the Enterprise crew starts exhibiting unusual behavior. Most obvious is Worf, who starts becoming almost feral in his behavior. Troi starts feeling excessively cold, Barclay is running around like he's on meth, and Riker is finding it difficult to concentrate.
The situation comes to a head when Worf enters Troi's quarters while she is bathing (in her uniform, no less) and bites her in a fit of primal lust. By the time the two of them are taken to Sickbay, Worf is no longer responding to questions. Crusher notices that he has developed some kind of toxin sac on his neck, and when she tries to examine it, Worf sprays acid directly in her face. The results are unpleasant to say the least.
With Crusher incapacitated, much of the crew sick, and a crazed, poisonous Worf on the loose, the remaining senior staff hold a meeting to discuss how to handle the situation. Unfortunately, most of them are too far gone by now, particularly Riker, whose cognitive problems have rendered him unable to command.
When Picard and Data return from their mission some days later, they find the Enterprise adrift and unresponsive to hails. Inside they find what appears to be a mostly empty ship, but internal sensors confirm that the the crew is present, and they have all started to de-evolve into various primitive species. Troi resembles an amphibian species, Riker has become proto-human, and Barclay arachnid. An airborne virus is causing random introns, latent DNA from previous evolutionary forms, to activate, causing the mutations.
Non-human species are also affected, including Spot. Picard notices that Spot has given birth and her kittens are not affected, leading Data to conclude that Spot's prenatal antibodies made them immune. To that end, they find Nurse Ogawa, since she is also pregnant, to acquire humanoid antibodies that will be effective for the crew.
There is one other problem: Worf's transformation has taken the form of a highly aggressive proto-Klingon predator. As he is trying to break into sickbay, Picard realizes that Worf is being driven towards Troi by his mating instincts. To give Data time to work on the antivirus, Picard lures Worf away using a hypospray containing Troi's pheromones.
After a chase through the corridors, Picard is able to improvise an electrical trap that incapacitates Worf long enough for Data's cure to take effect. Once the crew has returned to normal, Crusher finds that the virus came about from the injection she gave to Barclay, which had the unintended side-effect of activating more than just the one gene it was meant to due to a unique characteristic of his... something or whatever, you get the picture.
- Artistic License Biology: Put simply, the events of this episode take a lot of liberties with how evolution and DNA work.
- Artistic License Medicine: Crusher claims that new diseases are usually named after the first patient diagnosed with the disorder. Not only is that not strictly accurate in-episode, as Worf was actually the first person to be diagnosed with specific symptoms, the standard practice is for diseases to be named after the medical professional who first identifies them, meaning that the disease should have been called Crusher's Protomorphosis Syndrome. That said, it's quite understandable why she doesn't want her name associated with a boo-boo that resulted in an entire starship crew being mutated, one crewman ending up dead, and her own face being melted off.
- Asteroid Thicket: Actually Lampshaded for once—Data mentions that the asteroid field is "unusually dense," which is why Picard and Data use a shuttle to chase the stray torpedo.
- Beauty Is Never Tarnished: Barclay turns into a mutant many-eyed spider, Riker turns into an ape, and Worf into a completely unrecognizable crustacean-like monstrosity. Troi, by contrast, gets frog hands, amphibian eyes, and a little bit of green scales on her face and neck, but that's about it, and Crusher's disfigurement is never actually shown onscreen at all.
- Body Horror: Due to the gene activation injection, the crew starts transforming into various humanoid hybrids. This is not limited to humanoids; Data's cat Spot turns into an iguana. For example:
- Barclay: A spider
- Troi: An amphibian
- Riker: An Australopithecine
- Worf: A proto-Klingon predator
- Livingston the lionfish: A jellyfish
- Picard (though he doesn't transform): A lemur
- Nurse Ogawa: An unknown proto-human
- Chekhov's Gun:
- Spot's soon-to-be-born kittens and Ogawa's pregnancy provide Data with what he needs to reverse the de-evolution.
- Barclay hooks up a power conduit early on as a bypass. Picard uses it to shock the transformed Worf during the climax.
- Coitus Interruptus: The injury Riker's being treated for in the prologue came about when a date in the arboretum got hot-and-heavy, and he rolled into some cactus in the process.
- Continuity Nod: Isn't Barclay afraid of spiders? And that's what he turns into?
- Devolution Device: Albeit unintentional.
- Distracted by the Sexy: Worf leers inappropriately at the female server in Ten Forward as his baser instincts start to take over. She's not even wearing anything particularly sexy, but that doesn't seem to matter to him.
- Gone Horribly Wrong: Crusher's injection to activate Barclay's dormant immune gene accidentally ends up activating a whole lot more.
- Gory Discretion Shot: We don't see the effect of Worf's acid attack on Dr. Crusher, but her reaction makes it clear how painful it was, and Ogawa says she will require extensive reconstructive surgery.
- It's All My Fault: Worf blames himself for the torpedo malfunction.
- Jump Scare: Spider-Barclay jumps up and scares the hell out of Picard.
- Kindhearted Cat Lover: It turns out Barclay is a cat person. Though Spot's previous sitters have complained about what a terror she is, she and Barclay seem to get along just fine.
- LEGO Genetics: The intron virus and its effects are quite firmly outside the realm of plausibility. Apart from the token Technobabble needed to move the plot forward, no attempt is made to justify it.
- Motor Mouth: Barclay's first symptoms are twitchiness and rapid speech.
- Not Himself: Behavioral changes are the first symptoms of the intron virus.
- Nothing Is Scarier:
- An in-universe example with Picard, who starts to become abnormally anxious due to the intron virus ramping up his fight-or-flight instinct. Despite this, in true badass fashion, he manages to control his fear enough to not only come up with a solution, but provoke Worf into chasing him to buy time for Data.
- Speaking of Worf; the viewer is never given a clear look at what Worf turned into as a result of the virus. Whatever it is, though, has Picard scared out of his wits!
- OOC Is Serious Business: One of the early symptoms of the syndrome involves behavioral changes. For example:
- Riker's mind wanders, and he can't think straight.
- Barclay is hyperactive.
- Geordi is easily tired.
- Troi is constantly cold and thirsty.
- Worf is even more aggressive than usual (including sexually).
- Picard is on the verge of panic.
- Patient Zero: The injection Barclay received for his flu is what started the virus aboard Enterprise.
- Power Cable Attack: Picard uses this to incapacitate Worf. Bonus points for grabbing a non-conductive object to squat on while he lights up the Jeffries tube.
- Red Shirt: One is confirmed killed before he started transforming.
- Spiders Are Scary: Especially a six-foot spider who used to be a man.
- Super Spit: Worf shoots out a spray of acid that takes Crusher out of the episode early on and injures Picard during the climax.
- That's an Order!: Riker ordering Worf to take a break.
- Too Dumb to Live: Despite Worf acting extremely unusual, and Crusher noting that he's growing an acidic toxin sac on his neck, she stares him right in the face and tries to examine his mouth, getting a faceful of acid as a result. Real smart, doc.
- What Happened to the Mouse?: Geordi is among those affected by the mysterious illness, and his symptoms (extreme lethargy and becoming exhausted easily) are different than everyone else's, suggesting that he is devolving into a unique creature...but he's the only main crew member to not show up fully transformed in the final acts, so we never find out what exactly he becomes.