Lester's early career included a period of service in Starfleet. She spent a year alongside James T. Kirk and they became romantically involved. The perceived lack of opportunities for a woman to command a starship struck both Lester and Kirk as unfair, but she became embittered by the supposed career barrier. Their relationship soured to a point where Kirk felt she was punishing him for her circumstances. Years later, Kirk said "we'd have killed each other" if they had stayed together. Lester recalled that Kirk walked out on her "when it became serious".
Lester abandoned her career in space, and she nursed powerful resentments for the femininity that stood in her way, and for the masculinity that served Kirk so well. She eventually made a career for herself as a scientist and became a doctor, by title.
From 2267 to 2269, Lester was the leader of an expedition to Camus II, exploring the ancient remnants of an advanced civilization. Among the ruins, she discovered a sophisticated mechanism still in working order. The device was capable of life-entity transfer – the exchange of one being's complete personality and memory with another's. Her plans to use the device herself required several months of preparation. The expedition's incompetent physician, Dr. Arthur Coleman, bore an unrequited love for Lester. Together, they killed the expedition's entire staff by quietly exposing them to celebium radiation. The mysterious deaths announced by a distress call were meant to attract the attention of a starship, and the USS Enterprise answered the call on stardate 5928.5. (TOS: "Turnabout Intruder")
Lester as Kirk
Kirk arrived at Lester's bedside, where she appeared to be near death. Not expecting danger, Kirk was easily ensnared. Lester activated the life-entity transfer device, and was immediately thrilled by her possession of Kirk's body. The transfer would eventually return the exchanged persona to its original body, as long as both remained alive. Drugged and disoriented within Lester's body, Kirk was easy prey, but Lester's spontaneous gloating monologue prevented her from strangling Kirk before Dr. Leonard McCoy arrived.
Lester eagerly boarded the Enterprise as the Captain, satisfied to finally have the command she felt was rightfully hers. Kirk was still a liability as long as he remained alive in sickbay, but the exclusive care of Dr. Coleman kept Kirk isolated from the crew, while she put to test her months of study on every detail of a starship's operation.
Lester's attempt to emulate a Captain's behavior was marginally successful at first, but her composure degraded at any hint of scrutiny from the officers familiar with Kirk. Dr. McCoy soon challenged the Captain's mental health, and insisted on tests that ultimately proved negative for any illness. Spock discovered the truth of the situation through a brief mind meld with the imprisoned "Janice Lester", but their attempted escape was halted by Security officers ignorant of the Captain's strange new behavior.
"Captain" Lester convened a court martial, charging Spock with mutiny. Whatever support Lester believed she had, it all but evaporated during her farcical prosecution and hysterical closing rant. Once Lester announced the impending execution of Spock, the Enterprise crew was on the verge of actual mutiny.
Symptoms of the waning personality transfer forced Lester to make a final attempt to kill Kirk. The two touched in a brief struggle, and the misplaced personalities returned to their proper bodies. Broken, incoherent, and sobbing in her complete failure, Janice took her accomplice Coleman's arm and wanted him to kill Kirk. Coleman offered to help rehabilitate her with the assistance of McCoy, who escorted them both to sickbay. Kirk couldn't help but feel sorry for the poor, mentally unstable, twisted-minded woman, who was driven mad not only by her ambition of craving the power to command a starship, but also her hatred and jealousy of the captain she once loved, then wanted dead. Kirk felt her life could have been as rich as any woman's, "if only..." (TOS: "Turnabout Intruder")
Lester's remark to Kirk, that his world of Starfleet captains did not admit women to their ranks, implies the Starfleet culture of the 2260s was more sexist than its 20th, 21st, 22nd or 24th century counterparts. An interpretation offered by the Star Trek Chronology (2nd ed., p. 78) suggests Lester's comment referred to Kirk's inability to maintain a relationship, due to his responsibilities as a starship captain. It also states that Gene Roddenberry, on the other hand, admitted in his later years that the line was simply sexist.
Assuming that such blatant sexism was indeed policy or practice in Starfleet, it seems that by the 2280s and the 2370s, the bias against women commanding starships had been corrected. In the early scenes of Star Trek IV: The Voyage Home, a woman is shown to be in command of the Miranda-class USS Saratoga. Also, throughout the series Star Trek: Voyager, set a century after "Turnabout Intruder", Kathryn Janeway is shown to be in command of the USS Voyager. Additionally, Erika Hernandez was captain of Columbia NX-02 a century before "Turnabout Intruder", and Philippa Georgiou was captain of the USS Shenzhou over a decade before Lester's remark.
James Blish's novelization of "Turnabout Intruder" in Star Trek 5, Dr. Arthur Coleman's first name is "Howard" – probably from an early draft of the script. At the end of the episode, Kirk muses about Janice Lester, "Her life could have been as rich as any woman's, if only... if only...." In Blish's rendition, Spock finishes the sentence, adding: "If only she had been able to take pride in being a woman."