Lethe was a female Human civilian at the rehab colony on Tantalus V in the 23rd century. In 2266, she was committed to the colony for rehabilitative therapy. After being rehabilitated, she stayed at the colony as a therapist. Dr. Tristan Adams judged her to be a very good therapist. She had the same blank look as the other staff members and inmates of the colony; they had been victims of the neural neutralizer.
Questioned about her offense by Captain Kirk, she replied "Does it matter? That person no longer exists." She described her previous self as a "different person – malignant, hateful." Dr. Helen Noel remarked that the shifting of memory patterns was basic to psychotherapy; Dr. Adams added that patients should not be required to relive the evils committed earlier in their lives, when they were ill. She showed her loyalty to Dr. Adams by calling security on Dr. Noel. (TOS: "Dagger of the Mind")