Geordi La Forge’s friend and former crewmate, Lieutenant Commander Susanna Leijten, has come aboard the Enterprise-D; she is concerned that she and La Forge are the only two members of a previous 5-man away mission from the USS Victory to a seemingly abandoned colony on the planet Tarchannen III that are still accounted for; the other three, independently, have stolen a shuttlecraft from their current post and returned to the planet. Picard orders the ship to Tarchannen III; there, they find one of the stolen craft, manned by Lt. Hickman, attempting to land on the planet. The Enterprise is unable to communicate to Hickman, and the craft is incinerated during re-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 blood, 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 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 humanioid creature that created the shadow. It resembles what Leijten was transforming into. He suddenly goes into convulsions himself and soon transforms into the alien creature easily escaping the Enterprise due to his invisibility and beaming down 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 way the species has of reproducing. On learning that La Forge has left the Enterprise and beamed to the planet and that an away team is going to search for him, Leitjen says that she is the only one that will be able to find him. Leijten joins the away team on the surface searching for La Forge. 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 around the planet to protect both the creatures and the Federation.