Lethe was committed to the Tantalus Penal Colony during the mid-23rd century. By 2266, she had been rehabilitated and had elected to stay on as a therapist on Dr. Tristan Adams' staff.
Questioned about her offense by Captain Kirk, she replied "Does it matter?" 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. (TOS: "Dagger of the Mind")
Lethe was played by actress Susanne Wasson.
The Star Trek Encyclopedia speculates that the character was named after Lethe, one of the rivers of Hades. Mythology held that souls were made to drink from this river so they would not remember their past lives when they were reincarnated.