Five years later the members of the away team began to disappear one by one, apparently suffering an irresistible urge to return to the planet. The Enterprise-D was assigned to investigate; the ships chief engineer, Geordi laForge, had been a member of the Victory's away team and the final unaffected member, Susanna Leitjen, joined the ship for the mission.

Once at the planet both Leitjen and LaForge began to be affected by a mysterious metamorphosis which began to transform their entire bodies. Doctor Crusher discovered that they had both been infected by a strange parasite on the planet. The Parasite, which lives on Tarchannen, injects a strand of DNA into a host body. Years later the strand becomes active and begins to take over the host body, converting its DNA into an alien form. The host metamorphoses into a Humanoid lizard like form. The entity is non sentient, and has an instinctive fear of other life forms. Photoactive cells on the skin allow a chameleon like camouflage effect, rendering the entity almost completely invisible.

Doctor Crusher was able to remove the DNA from Leitjen, but LaForge beamed down to the planet under its influence and joined the others on the surface. The Enterprise crew was able to find a method of rendering the aliens visible, and with this Leitjen was able to appeal to the remaining shreds of LaForge's Humanity and convince him to return to the Enterprise. He was also subsequently cured, and Captain Picard placed warning beacons in orbit of the planet to prevent any further contact.