The site of a Federation penal colony run by Dr. Tristan Adams, where an experimental neural neutralizer is secretly misused in Dagger of the Mind.
Tantalus V is home to a Federation rehabilitation colony for the criminally insane, regarded as a model of humane treatment under its director, Dr. Tristan Adams. The colony is protected by a force field and can be reached only by transporter.
In Dagger of the Mind, the inmate Dr. Simon van Gelder escapes to the Enterprise in a cargo container and warns of abuses. Kirk and the psychiatrist Helen Noel beam down to investigate and discover that Adams is using a device called the neural neutralizer to empty and reshape minds. Spock's mind meld with van Gelder exposes the truth.