Geordi La Forge’s friend and former crewmate, Lieutenant Commander Susanna Leijten, has come aboard the Enterprise-D. She is concerned because of a previous mission, in which she, Geordi, and three crewmates from the USS Victory were part of an away mission to the planet Tarchannen III; their crewmates have all deserted their posts, stolen shuttlecrafts and attempted to return, leaving Leijten and La Forge as the only ones unaffected. Picard orders the ship to Tarchannen III; there, they find one of the stolen craft, manned by Lt. Hickman, attempting to land. The Enterprise is unable to establish communication with Hickman, and the craft is incinerated during atmospheric entry.

On the surface, Leijten and La Forge discover another shuttlecraft, empty; strange footprints are seen near it. Leijten starts feeling ill, and they return to the Enterprise. Leijten improves immediately, but Dr. Crusher finds evidence of alien skin in Leijten's body, not indigenous to the planet. While reviewing the data on the original Victory away team, Leijten becomes jittery and insists they return to the planet, and then starts exhibiting bright blue veins across her body. In Sick Bay, Dr. Crusher restrains Leijten and determines she is transforming into another species, and worried that this fate will also befall La Forge. La Forge continues to study the data and discovers an anomaly in the original mission data. He notices a shadow in the recording that does not appear to belong to anyone on the mission. Starting to feel the same effects as Leijten, La Forge uses the data in the holodeck to reproduce the scene. By eliminating members of the mission from the scene he manages to discover that their team was in the presence of an invisible alien humanoid creature that created the shadow. It resembles what Leijten was transforming into. He suddenly goes into convulsions himself and soon transforms into an alien creature himself. Due to his newfound invisibility, Geordi easily evades the Enterprise sensors and transports to the surface.

Dr. Crusher discovers that Leijten's transformation was triggered by an alien parasite that was capable of overwriting its host's DNA; removing the parasite quickly returns Leijten to normal. Leijten tells Dr. Crusher that placing the parasite in the host is the aliens' method of reproduction. Leijten joins the away team on the surface searching for La Forge, because (having previously suffered the transformation) she will be able to track him. Using ultra-violet light, they discover a number of the creatures, all transformed members of the colony and the former Victory crewmembers. Leijten recognizes one as La Forge, and is able to coax him away and take him back to the ship where he is similarly cured. However, the other crewmen have been transformed far too long to be saved, and they are left behind on the planet; Picard orders warning beacons placed in orbit to protect the creatures (and prevent more victims from being transformed).

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