Created by Gene Roddenberry, the science fiction television series Star Trek (which eventually acquired the retronym Star Trek: The Original Series) starred William Shatner as Captain Kirk, Leonard Nimoy as Mr. Spock, and DeForest Kelley as Dr. Leonard "Bones" McCoy aboard the fictional Federation starship USS Enterprise. The series originally aired from September 1966 through June 1969 on NBC.
This is the first television series in the Star Trek franchise, and comprises 79 regular episodes over the show's three seasons, along with the show's original pilot episode, "The Cage".
After the show's cancellation, Paramount Television released Star Trek to television stations as a syndication package, where the series' popularity grew to become a "major phenomenon within popular culture." This popularity would eventually lead the Star Trek franchise to expand its catalog to include five more television series and twelve Trek motion pictures.
On stardate 5928.5, the Federation starship USS Enterprise rushes to answer a distress call from the planet Camus II, the site of an archaeological expedition that is exploring ancient ruins of a long vanished culture. Upon arrival, the landing party finds three survivors; among them are Dr. Janice Lester and Dr. Arthur Coleman. It is revealed that during their academy days, Dr. Lester and Captain Kirk were intimately involved; however, Lester's inability to achieve command of a starship, which she believes had to do with her gender, caused the two to drift apart.
Medical scans reveal that the survivors are suffering from radiation exposure which threatens their lives. Dr. Coleman, Science Officer Spock and Chief Medical Officer Dr. McCoy go off to tend to the other survivors, leaving Lester and Kirk alone for a moment to talk. Kirk finds Lester has been driven to the point of madness by her personality complex and complains about the agony of being a woman in a male-dominated existence. She traps Kirk in an alien device which can swap the personalities between two individuals. Joining Kirk in the machine, Lester's essence is transferred into Kirk's body while Kirk's mind enters hers. Now as Kirk, Lester-in-Kirk tries to strangle her former body, inside which Kirk is trapped.
Spock and McCoy return before Lester-in-Kirk can finish him off, and she (as Kirk) orders the landing party and the remaining survivors back to the Enterprise. Once there, Lester-in-Kirk releases McCoy from Lester's case and gives Dr. Coleman, Lester's partner in crime, full authority for treatment. McCoy protests the decision (later telling Lester-in-Kirk that Coleman is known to have been ruled as incompetent to serve aboard a starship by the Starfleet surgeon general; the "captain" asserts that promotions and demotions can be politically motivated), but McCoy stands his ground with an ornery tone of voice.
Alone, Coleman and Lester-in-Kirk privately converse about their plans. It is revealed that the two are in love with each other and had planned to have their personnel killed in order to lure the Enterprise to Camus II. Lester-in-Kirk has now achieved her lifelong goal of commanding a starship. Likewise, Coleman has now been given a chance to become a starship's chief medical officer. Lester-in-Kirk charges Coleman to make sure that Kirk-in-Lester, who is still trapped inside Lester's female body, stay out of the way by sedating him.
Spock becomes suspicious when Lester-in-Kirk orders a course change to the Benecia Colony to drop off Dr. Lester for medical attention, ignoring their previous mission; to rendezvous with the USS Potemkin at Beta Aurigae. Spock vainly points out that Benecia's medical facilities are comparatively primitive, and they are on course to adequate facilities, which they can reach at maximum warp in a time comparable to reaching Benecia. McCoy invokes his authority to override his "captain"'s orders and perform a full medical examination, including tests that will check for changes in his basic personality traits and responses, the latter test Lester-in-Kirk being leery of but seems to pass.
Kirk-in-Lester manages to escape and goes to sickbay for help, but finds Lester-in-Kirk still there being given a bill of health by McCoy. Lester-in-Kirk uses a series of violent slaps to render her "former" body unconscious, which McCoy demands he stops. Lester-in-Kirk orders the "hysterical woman" to be taken into custody and put into isolation: "no one is to speak to her without my permission".
Kirk-in-Lester is taken away, and Spock persuades the guard that the captain's orders have never been meant to apply to his senior officers and, in this case, keep Spock from speaking with "Lester". Spock speaks with "Lester", is somewhat skeptical, then Kirk-in-Lester persuades Spock to touch him with the Vulcan mind meld, whereupon he discovers the truth that Captain Kirk's mind is indeed present.
Spock tries to free Kirk-in-Lester but is stopped by a security team led by the impostor captain. Lester-in-Kirk accuses Spock of mutiny and immediately (and flamboyantly, via ship-wide announcement) orders a court-martial. Once on trial, Spock tries to make a convincing case that Captain Kirk is really in the body of Dr. Lester, and that the physical Kirk is an impostor. However, his evidence was obtained by telepathy, still considered a form of hearsay in the 23rd century, and is inadmissible.
Shocked by the incredible story, McCoy and Chief Engineer Scott converse secretly in the corridor about their Captain's true identity and having witnessed Kirk's irrational behavior, and hostility toward Dr. Lester, they decide as members of the court martial panel to agree with Spock. Scott says that the "captain" won't let the decision stand, and suggests that he and McCoy "move against him" to relieve the "captain" of command. McCoy is reluctant to the mutiny, but agrees it's the only way. Lester-in-Kirk suspects they spoke in the corridor for a reason, and orders the court recorder to call up the conversation from recordings of corridor activity. Lester-in-Kirk then orders McCoy, Scotty, Spock and Kirk-in-Lester (still in her former body) arrested for treason. Their punishment will be death, and interment of the bodies will be at Benecia. Ensign Chekovand Lt. Sulu are outraged and point out that Starfleet bans the death penalty except for "General Order 4", which isn't the case here. Lester-in-Kirk refuses to listen and demands that everyone return to their posts.
The remaining crew quickly turn against their "captain" when they realize that he isn't himself and refuse to obey his orders. Losing self-control and throwing a fit because Chekov and Sulu blatantly ignore her orders and in unison take their hands off their consoles, Lester-in-Kirk falls into the center seat and she and Kirk-in-Lester have a temporary reversion of minds. Kirk-in-Lester determines to fight for his own body if it happens again. Lester-in-Kirk hurries to Coleman and tells him of the incident; Coleman tells Lester-in-Kirk that to make the transfer permanent, she must kill her former self while Kirk still occupies the body. Lester-in-Kirk shames and blackmails Coleman into prepare an injection of a toxic substance.
The two head to Lester's holding cell, but Lester-in-Kirk again loses self-control when she sees Kirk-in-Lester struggling with Coleman as he vainly tries to inject the toxin, and the reversion starts again. This time Lester is too late, and the reversion completes. Captain Kirk is finally himself again. The hysterical Lester makes one last attempt to kill Kirk but fails, and Lester and Coleman are taken into custody. Coleman then pleads with Kirk to allow him to care for his mentally deranged love, and Kirk decides that they will be dropped off at Starbase 2 to face charges. In the meantime the Enterprise resumes its current mission with the Potemkin. Kirk muses, "Her life could have been as rich as any woman's. If only... if only..."