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Talosians are the second alien race filmed (but not initially aired) in the history of Star Trek. They first appeared in the originally unaired pilot "The Cage," and then again in "The Menagerie" (which was comprised of a majority of the footage from the pilot).
Talosians were a race of humanoids from Talos IV, which they called Clesik. Their distinguishing feature were large craniums that contained brains three times the size of Vulcan or human brains.
The Talosian race developed warp capability half a million years ago. Thousands of centuries before the mid 2200s the Talosians fought each other in a series of wars that left Talos IV desolate and incapable of sustaining life. It was only in the 23rd century that the planet began to recover.
As a result, the Talosians retreated underground and developed their psionic abilities. However, the use of their abilities to create illusions became addictive. Finding illusion preferable to facing real life the Talosians spent nearly all their time in illusions, forgetting how to fix the machines left by their ancestors and making little effort to repair their devastated world.
After many thousands of years their mental abilities increased to the point that the Vulcan Spock once postulated that they could reach out and swat a Constitution-class starship as if it were a fly just by using those mental powers. By the 23rd century the Talosians mostly communicated telepathically but were still capable of speech.
With the number of Talosians took to abducting members of other species to determine if those species could be adapted for life on Talos, and serve the Talosians to rebuild their race.
In 2236 the Talosians encountered their first human when the SS Columbia crashed on their world, leaving a sole survivor named Vina. They attempted to repair their injuries as best they could but due to their unfamiliarity with human anatomy their treatments left her disfigured. Providing her with the illusion of a healthy and young body, Vina lived with them for the next 18 years.
Encounter with the Enterprise
In the year 2254, the Enterprise, under the command of Captain Christopher Pike, received what appeared to be 18 year old distress signals from the deep space ship Columbia, which crash landed on Talos IV. Both the initial and follow up signals were faked. Upon investigating the Enterprise crew discovered the survivors they saw were only an illusion to lure the crew to the planet.
Led by their Magistrate, the Talosians abducted Captain Pike and held him in prison and attempted to artificially create a romantic attraction between him and Vina by inserting her into a re-enactment of a combat incident Pike had previously had on Rigel VII. Once the battle was over, Pike began asking Vina about the Talosians and their intentions. Finding herself at a loss for words, Vina was told by the Talosians to back off so they could regroup, leading Pike to believe that Vina was taken to be telepathically punished for overstepping her bounds in revealing too much and attempting to seduce Pike prematurely, Pike realized that Vina was real, as opposed to yet another illusory creation he previously suspected her of being.
When Pike refused to consume the nourishment the Keeper punished him with a vision of hell. After drinking the liquid, Pike then surprised the Keeper by allowing himself to become completely angry while attempting to break through the transparent barrier. Pike learned that the Talosians could not read through strong emotions. The Keeper changed the subject to reveal their rationale for abducting Pike: As the sole survivor of the Columbia crash, the Talosians intended to use Pike and Vina in their plans to create a human community to serve as slaves to the Talosians.
Taking his new sympathies towards Vina as a sign of progress, the Talosians proceeded to deepen Pike's relationship with Vina through two other environments: one at Pike's ranch in Mojave, California, and another at an Orion slave colony (with Vina portrayed as an Orion slave girl). Pike resisted, however, and the Talosians abducted two female Enterprise crew members as alternative mates for Pike: Number One and Yeoman J.M. Colt. Pike resisted these efforts as well, eventually managing to escape by shooting a hole in the cell transparency (and threatening to likewise shoot the Keeper if he didn't remove the illusion that he had failed to do so).
Once they arrived at the planet surface, Pike offered to stay with Vina if the Keeper would allow Number One and Yeoman Colt to return to the Enterprise. Number One started to overload her phaser, which would have killed all of them in order to prevent the Talosians from breeding Pike and Vina together and using their offspring as slaves.
Conferring together, the Talosians reviewed the records of the Enterprise and learned that humans had a unique and violent hatred of captivity. Even though no other species had shown the same adapatbility as humans, their hatred of captivity made humans unsuitable to be kept on Talos. Learning that the Talosians would eventually become extinct, Pike offered Federation assistance, but the Talosians refused, fearing that contact with humans would lead humans to develop the same abilities the Talosians had and destroy themselves in the process.
Number One and Colt were returned to the Enterprise, but Pike remained just long enough to learn Vina's rationale for remaining - she had been severely disfigured in the Columbia crash, and although the Talosians were able to repair her injuries to keep her alive, they were unable to restore her appearance except through illusory means. The Talosians allowed her to remain with a telepathic copy of Captain Pike, and the real Pike was allowed to return to the Enterprise.
Return to Talos IV
In late 2257, a disabled Spock kept repeating the coordinates of Talos over and over, but was doing so backwards. Realizing that Spock had regressed to a point in his life where he had a learning disability, Michael Burnham reversed the order in which Spock was repeating the coordinates and realized he had given the coordinates to Talos IV. Feeling that the answers to help Spock were on Talos IV, Burnham took Spock to that world.
Some time after Burnham and Spock visited, Pike's account of the mission led the Federation to issue General Order VII, forbidding any visit to Talos IV under any circumstances. In the mid 23rd century violation of the order was the only one still punishable by death in the Federation.
When Pike suffered his own crippling injury, however, the Talosians learned of this and contacted Lt. Cmdr. Spock and requested he bring Captain Pike to them. Against Pike's wishes, Spock commandeered the Enterprise (now under the command of James T. Kirk) and used it to transport Pike from Starbase 11 back to Talos IV. Spock and the Keeper staged a fake court-martial for Captain Kirk and Commodore José Mendez to recount the events of the original Talos IV mission (and to stall for time to allow the Enterprise to arrive at Talos). Mendez from his starbase learned what the Talosians intended and suspended General Order VIi just for the transportation of Captain Pike to Talos IV. Upon entering orbit, the Magisgrate telepathically communicated with Captain Kirk to invite Captain Pike to spend the rest of his days with Vina and the Talosians. Pike accepted the invitation and was telepathically granted the illusion of full health and strength.
Even though they were largely isolated from the galaxy, the Talosians were able to keep abrest of galactic events and kept Pike informed on how his former shipmates were doing.
In the years that followed with Pike's assistance the Talosian race finally began taking steps to reclaim their homeworld. In the early 24th century Vina died due to her advanced age, and Pike followed soon afterwards. Pike had the Magisgrate contact Spock to summon him to Talos IV to witness the significant progress the Talosians had made in rebuilding their world. In place of a sickly, desolate world Spock found a thriving world with less ambient radiation than that present on 24th century Earth. He found that the Talosians had rebuilt their cities and moved above ground, and had learned how to limit their time spent in illusions.
The Talosians played the message Pike had left for him, asking him to go to the Federation to advocate for the lifting of General Order VII and for Talos to join the Federation. Additionally, Pike requested Spock return some of Pike's ashes to Earth for burial.