Original air date: March 25, 1991
On Tarchannen III, Lt. Cmdr. Susanna Leijten leads an away mission with Starfleet officers Hickman, Brevelle, Mendez, and a young Lt. La Geordi Laforge, still in a red uniform. They're investigating the disappearance of 49 colonists. Since this event, two of these officers have themselves disappeared mysteriously, and the third, Hickman, has stolen a shuttlecraft to return to the planet. On the Enterprise, Leijten briefs the bridge crew as the ship journeys to intercept Hickman. On the way there, she and Geordi reminisce about their time serving together, revealing that Leijten has been a big sister to Geordi.
The crew arrives just in time to see Hickman barreling his stolen shuttlecraft recklessly into the planet. Picard tries to talk him down, but the fugitive burns himself up in the atmosphere. Riker leads an away team to the planet to get some answers, with Leijten and Geordi in tow. The officers locate another stolen shuttlecraft and the tattered remains of a Starfleet uniform, evidence that Mendez had arrived here. While exploring the area, Geordi discovers Leijten having some sort of psychotic break and has her beamed back to Sick Bay.
Crusher notes that Leijten is having a histamine reaction to biological material from the planet and attempts to treat her. Leijten remains jittery and becomes increasingly fixated on returning to the planet. She begins to have another psychotic break, causing her to collapse into Geordi's arms, where he notices that she's physically transforming into an alien form. In Sick Bay, Crusher confirms that Leijten is transforming into an alien, one whose skin is highly sensitive to light and reacts to it with radiance of its own. Crusher tells La Forge that his own transformation is likely imminent.
La Forge decides to spend what time he has left as a human studying the mystery on the planet. He simulates the away mission's recording in the holodeck and discovers that a human-shaped shadow was standing right next to them the whole time. Meanwhile, Crusher has located an alien parasite in Leijten's body, the source of the transformation. She removes it, and Leijten begins to recover. Leijten says that the parasite is how the aliens procreate. They try to locate Geordi to have his own parasite removed, but Geordi is nowhere to be found.
The Chief Engineer's transformation has already completed. The Geordi-Alien turns invisible and beams itself down to the planet. An away team is sent to recover him. Using UV lights brought by Data, they manage to locate the invisible aliens. Leijten leverages her relationship with Geordi to contact the small remnant of himself still lingering in the alien body. He embraces Leijten and allows himself to be returned to the ship.
After getting his parasite removed, the newly restored Geordi states that within a few minutes, his mind would have been completely lost. The former Starfleet officers and colonists are irrevocably transformed, so Picard has warning beacons placed around the planet and leaves them in peace. Geordi thanks Leijten for all of her big sisterly help.
Tropes featured in this episode include:
- Author Appeal: Writer Brannon Braga seems to have a thing for humans mutating/evolving into alternate life forms; he also wrote the seventh season's "Genesis", as well as the infamous Voyager episode "Threshold". And was also executive producer on the short-lived series Threshold.
- Baleful Polymorph: The parasites injected into the humans begin re-writing their DNA, slowly mutating them into something non-human.
- Bittersweet Ending: Geordi and Leijten are safe, but the other two crewmen can never be returned to normal and Hickman died trying to get back to the planet; Geordi mentioned previously that he had a family, and now they'll never see him again. The most the crew can do is leave them in peace and set up a warning marker.
- Body Horror: Oh, yeah. We see the beginnings of Geordi's transformation, and he's quite horrified and pained by it.
- The Cameo: Brevelle and Mendez in their mutated forms were portrayed by Mark Thompson and Brian Phelps, the hosts of the popular radio talk show The Mark and Brian Show.
- Casts No Shadow: Inverted; the normally invisible creatures do cast shadows.
- Continuity Nod: For the scenes set in 2362, the Victory crew are wearing the one-piece jumpsuits and carrying the older "dustbuster" phasers. Geordi is also wearing an older-model VISOR as well as a red uniform, like he did in season 1 before he transferred to engineering.
- Enhance Button: Executed somewhat more plausibly than the norm. In order to determine the source of a particular shadow in a video log, Geordi has to feed the holodeck computer a series of assumptions and speculations—and even then, the result is somewhat inconclusive (as seen above).
- Face Full of Alien Wing-Wong: The Tarchannen III aliens reproduce by implanting a parasite that transforms other organisms into a member of their species. This happens to the inhabitants of a Federation outpost and several Starfleet officers sent to investigate.
- Foreshadowing: When the Enterprise crew beams down the first time, they find the shuttles Mendez and Brevelle stole along with a pair of torn Starfleet uniforms.
- "I Know You're in There Somewhere" Fight: Leijton to the Alien-Geordi. Reversed and Downplayed earlier with Geordi to a slightly-mutated Leijton.
- Invisibility: The creatures are invisible in normal light (though they do cast shadows), but can be seen using ultraviolet light.
- Invisibility Flicker: When illuminated by blacklight. A more brief instance is when an invisible Geordi uses the transporter.
- Let's Split Up, Gang!: When Leijten goes missing on the planet, the other members of the away team split up to look for her. Nobody seems to worry that this makes them vulnerable to being picked off one by one.
- Like Brother and Sister: Geordi's relationship with Susanna Leijten clearly has this dynamic.
- Loss of Identity: The end result of the Viral Transformation. The creatures that remain are nothing more than instinctive animals. Geordi remarks that he was only a few minutes away from completely losing his humanity.
- Painful Transformation: The initial symptoms include tremors, panic, and the fusing together of the victim's fingers. Then the real pain kicks in.
- Send in the Search Team: What kicks off the whole plot—the Victory was sent in five years ago to find out what caused the entire crew of an outpost to disappear.
- The Shadow Knows: When Geordi starts reviewing the Victory's recording in the holodeck, he notices a shadow that doesn't belong to any of the people or objects present. He activates the computer's Enhance Button, but all it can do is project a vague shape of the source.
- Shapeshifter Mode Lock: Of course, once the human DNA has been completely overwritten, there's no going back.
- Shapeshifting Excludes Clothing: See Wham Shot, below.
- Viral Transformation: How the entities on Tarchannen III reproduce—they infect visitors with a parasite that eventually compels them to return to the planet and slowly mutates them into another creature.
- Visible Invisibility: The Geordi-Alien employs the Predator Cloak.
- Was Once a Man: Mendez and Brevelle are too far gone to be saved by the time the Enterprise figures out what the cause is.
- Wham Shot: When Riker, Worf and Data go looking for LaForge on the holodeck. Worf calls out, then holds up LaForge's torn uniform and VISOR.