In 2266, Lethe 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 a blank look which the other staff members and inmates of the colony also had; they had been victims of a neural neutralizer that Dr. Adams was experimentally using there.
Questioned about her offense by Captain Kirk, Lethe 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. Lethe showed her loyalty to Dr. Adams by calling security on Dr. Noel. (TOS: "Dagger of the Mind")
Lethe was played by actress Susanne Wasson.
In the final draft script of "Dagger of the Mind", Lethe was described thus; "She's a tall, handsome (if slightly cadaverous) young woman [....] Although she isn't an automaton, there is just a slight lack of normal Human spontaneity in both manner and voice. She wears what we will come to know as a 'Technician' version of the Tantalus Colony garb, similar to that worn by Dr. Adams himself." In actuality, she was the only individual "employed" at the facility who seems not to wear the so described the aforementioned garb.
Taking notice of the episode writer, Shimon Wincelberg's fondness of Greek mythology, with references to "Tantalus" and "Lethe", it was noted in Star Trek: The Original Series 365 (p. 073) of this character "Her blank expression and eerily emotionless demeanor suggest that Adams has wiped out more than her criminal tendencies with his machine. In Greek mythology, Lethe was a river in Hades: anyone who drank from it experienced complete forgetfulness. The name is the source for the English word "lethargy," meaning sluggish or indifference. Let we forget." Additional references, the Star Trek Encyclopedia (4th ed., vol. 1, p. 460) and The Star Trek Compendium (4th ed., p. 42), also relate this same general information.
Lethe was spoofed as "Little Missy" in the South Park episode "Roger Ebert Should Lay Off the Fatty Foods", which itself was a parody of "Dagger of the Mind". Missy utters Lethe's line, "I love my work" when introduced by Dr. Adams.