Stardate 44664.5: An away team from the USS Victory visited the planet Tarchannen III five years ago, to investigate the disappearance of the crew of an outpost. Two of the away team members have mysteriously returned there and vanished themselves without a trace. A third one, Lieutenant Hickman, is killed when his shuttle breaks up in the atmosphere of Tarchannen III. Now only Susanna Leitjen and Geordi are left. After a new away mission on Tarchannen III Leitjen's blood chemistry is altered and she feels the urge to return to the planet. Beverly suspects that the other crew members have been transformed into a species indigenous to the planet. She finds and successfully removes from Leitjen's body the parasite that is responsible for the genetic change. Geordi reconstructs the old away mission on the holodeck, but soon his transformation begins, and he leaves the ship. Down on the planet, Geordi has been almost completely transformed and can only be found using ultraviolet light. Susanna succeeds in convincing him to come back.

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