Enterprise stops at the Tantalus penal colony to deliver drugs and collect research materials. Except that the research materials box contains a dangerous madman, Simon van Gelder - the former director of the colony. Kirk and psychiatrist Helen Noel beam down to speak with Tristan Adams, to learn what happened to van Gelder. It seems Dr. van Gelder was the victim of a new device, a neural neutralizer. But was what happened really an accident, or something more sinister?