Geordi's friend and former shipmate, Susanna Leijten, informs him that the two of them are the only crew members remaining from an Away Team that once investigated the mysterious disappearance of several people on the planet Tarchannen III. The last of their former colleagues has stolen a shuttlecraft to head back to the planet, and Susanna enlists the U.S.S. Enterprise's aid to find out what is happening. The starship follows the shuttlecraft to the planet discovering two additional shuttlecrafts on the surface, but no life signs. After Geordi finds two torn Starfleet uniforms, Susanna tells him she senses the presence of the others and heads off into the darkness. When Geordi tries to stop her, she begins thrashing like a wild animal. He has her beamed to Sickbay.
Dr. Crusher finds that Susanna's blood chemistry has been altered, and Geordi notices that her hands shake uncontrollably. Susanna realizes that whatever happened to the other Away Team members is happening to her and could soon happen to Geordi. Confined to the starship so that Dr. Crusher can observe and treat her, Susanna begins to feel confined, and eventually rushes off to escape the ship. Before she can, however, she collapses. Geordi runs to her, discovering that her skin is covered with dark blotches and her two middle fingers have fused together.
Based on her examination of Susanna, Dr. Crusher determines that the people who disappeared from Tarchannen III were not abducted, but transformed into another species. While Beverly searches for the cause of Susanna's transformation, Picard issues an order that Geordi's every move be monitored by computer in case he should also be motivated to leave the ship. Geordi spends his time aboard ship painstakingly reviewing a visual recording made five years ago at the scene of the original investigation.
While reviewing the tape, Geordi notices a shadow that has no known origin. He recreates the entire scene in the Holodeck, determining that an unknown creature was present on the planet during the fateful Away Team mission. Moments later he is struck with a sudden pain, and looks down to see that his two middle fingers have fused together. Meanwhile, Dr. Crusher locates and removes the parasite responsible for Susanna's transformation. Hoping to scan Geordi for the parasite before it affects him, Dr. Crusher summons Geordi, but a security team discovers his torn uniform and sees a half-alien Geordi overcome a transporter technician and beam to Tarchannen III. Susanna insists on accompanying the Away Team on their search for Geordi, insisting that what she has gone through will help her find him. She is allowed to go, and finds him almost completely transformed. However, she is able to appeal to his human side and talk him into returning to the starship, where he undergoes surgery and regains his human identity.