Geordi La Forgeâs friend and former crewmate, Lieutenant Commander Susanna Leijten, has come aboard the Enterprise-D with a harrowing tale. Five years previously, she had led an away team from the USS Victory down to the planet Tarchannen III to check out a lost Federation colony, but she found no traces of the colonists. Recently, three of the five members of that survey team have stolen shuttlecrafts and headed back to the planet. Each of them did this independently, with no known motive for their actions. The remaining two members of the team are Leijten and La Forge.
Picard orders the Enterprise to Tarchannen III, and the ship catches up to one of the shuttles, piloted by Lt. Hickman. Hickman refuses to answer any communications and is incinerated during a botched entry into the planetâs atmosphere. Commander Riker takes Worf, Data, La Forge, and Leijten down to the surface to investigate, and they locate one of the other missing shuttles. While searching, Leijten discovers strange footprints and then starts acting very strangely herself, which prompts the team to immediately return to the Enterprise.
Once returned to the ship, Leijtenâs behavior normalizes, but Doctor Crusher does find some anomalies in Susannaâs blood chemistry. Data's analysis of a uniform they found in the shuttle reveals traces of alien skin, and Data also determines that the footprints Leijten discovered were not made by any indigenous creature.
La Forge and Leijten decide to try looking for some common link between the members of the original Away Team. However, while the investigation continues, Leijten grows anxious and insists that she and La Forge immediately beam back down to the planet to look for answers. When La Forge disputes this course of action, Susanna suddenly goes into convulsions and bright blue veins appear on the back of her neck. In addition, the first three fingers on each of her hands fuse together. Doctor Crusher determines that Leijten is somehow being transformed into another species, and warns that this could soon happen to La Forge, as well. Soon, La Forge does develop the same symptoms, but he continues to work and finally notices an anomaly in the recording from the original survey mission.
Going to the holodeck to create a simulation of the events, La Forge discovers an extra shadow in the visual record, one which cannot be correlated with any visible person or object. Using the computer, he manages to construct an approximation of the size and location of an apparent creature that cast the shadow, but suddenly convulses as his transformation accelerates. His humanity waning, he gives in to an intense urge to return to the planet below. Geordi has now mutated to the point where he cannot be located by the shipâs sensors, and so easily escapes pursuit aboard the Enterprise and beams down to the surface.
Doctor Crusher, meanwhile, has succeeded in tracking down the culprit - an alien parasite which spreads genetic construction to transform its host into another species - and has removed it from Leijten's body. Leijten joins the Away Team to search for her former crewmate. Using ultraviolet light, they manage to locate La Forge and several other similar creatures â former Victory crewmembers who suffered a similar fate. Leijten manages to break through to La Forge and bring him back to the ship; unfortunately, none of the other mutated humans can be recovered. With La Forge returned and recovering, Picard orders that warning beacons be placed on and around the planet to protect both the Federation and the planet's creatures.