LaForge's friend and former crewmate Susanna Leijten visits the Enterprise with alarming news: the members of an away team of which they were once members have been disappearing one by one, returning to Tarchannen III, where one of the Starfleet officers has either fled or been abducted in a shttlecraft. Picard takes the Enterprise in pursuit of the shuttle, but is unable to stop the frantic pilot from destroying the craft in the atmosphere. On the surface, the crew discovers the ships that brought the other landing team members to the planet, but no indication of what happened to the officers, nor any human life signs. Yet Leijten says that she can sense the presence of the others, and when LaForge pursues her, she panics. Back on the Enterprise, Crusher discovers that Leijten is undergoing a physical transformation, though there was no sign of the change when she came aboard the ship. Leijten tries to help LaForge search their old away mission records for clues about what is happening to her and the others, but she has trouble resisting the impulse to return to the planet. Rather than confine LaForge to sickbay in case the transformation begins to affect him as well, Picard orders the computer to monitor his movements. While Crusher determines that Leijten's DNA is being rewritten into that of an unknown species, LaForge reviews the records from the previous away mission and spots a shadow whose source he cannot identify. He recreates the scene on the holodeck, but before he can guess the identity of the unknown intruder, his fingers begin to fuse together. By the time Crusher finds and removes the parasite transforming Leijten's DNA, LaForge has become invisible to human eyes. The computer can no longer identify him, so he is able to override the transporter lockout and flee the ship. A recovering Leijten follows him to the planet's surface, where she is able to find him and persuade him that he is still human if only he will come back with her. He takes her hand, and she is able to beam him to sickbay, where Crusher restores his human DNA.
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