Geordi's old friend begins to change into something else.
Geordi La Forge is asked to look into the disappearance of members of an old away team. He is reunited with Susanna Leijten, who was a member of that team, and who is now a researcher. Soon the survivors begin to undergo a mysterious physical change, and the crew realizes that the transformation threatens Geordi too.
The story has an eerie, horror-influenced tone, building from a puzzle about a past mission on Tarchannen III into a chase through shadows. It gives Geordi a larger role and is one of the more atmospheric entries of the fourth season. Dr. Crusher's investigation and Data's logical thinking help to solve the case, and the ending is both tense and sad.