The survivors of the nuclear war congregated in underground dwellings, where they became dependent upon their own mental powers, which they used to create stunningly real illusions, an ability that had been developed by their ancestors. As their mental powers grew, they lost the ability to use the technology left behind by their ancestors.
The Talosians found that life using illusion was addictive, almost like a Human developing a physical and psychological dependence on narcotics. They became bored with the content of the illusions which they had. Their dependence upon these illusions for mental stimuli caused the Talosians to begin capturing space travelers to use as the living basis for their illusions.
In 2236, the SS Columbia, carrying members of the American Continent Institute from Earth, crashed on Talos IV. All aboard were killed, save for one Human: a badly injured female named Vina. The Talosians repaired her injuries, but their work left her disfigured, as the Talosians were unfamiliar with Human anatomy. Using their powers of illusion, Vina could live as if she was uninjured and was made to appear abnormally beautiful.
In 2254, the Talosians captured USS Enterprise Captain Christopher Pike and attempted to use him to rebuild their civilization. The Talosians hoped that Pike would be attracted to Vina and would wish to remain on Talos IV. Thousands of the Talosians probed Pike's thoughts, discovering he had "excellent memory capacity." However, after assimilating the records of the Enterprise, the Talosians learned that Humans have a "unique hatred of captivity;" even when made as pleasant as possible, Humans prefer death. This made Humans unsuitable to the Talosians for breeding stock, and Pike and his crew were released.
The Talosians refused Pike's offer of trade and mutual understanding, claiming that Humans would use their powers of illusion to their own destruction, as the Talosians had inflicted on themselves. After Vina's true appearance was revealed, she was given back not only her illusion of beauty, but an illusory Pike to keep her company. (TOS: "The Cage")
In 2257, Christopher Pike's science officer, Lieutenant Spock, began experiencing time non-linearly after an encounter with the being known as the Red Angel. Spock exhibited symptoms of a mental disorder, but was able to repeat the coordinates of Talos IV backwards, which he recalled from his previous visit with Pike. Brought to Talos IV by his adoptive sister Michael Burnham, the Talosians and Vina agreed to help heal Spock, and later assisted in their escape from the planet by projecting illusions on to the Section 31 ship NCIA-93, making Leland think that he had beamed them aboard while Burnham and Spock had actually escaped in a shuttlecraft. (DIS: "If Memory Serves")
This same compassion was again shown ten years later, when the Talosians collaborated with Pike's former science officer, the now-Commander Spock, to bring Pike back to Talos IV (even though Spock's participation in this effort involved defying Starfleet general orders and illegally taking command of the Enterprise). As Pike was himself a prisoner of his own body after an accident involving delta rays, the Talosians assisted with his return so he could live out the rest of his life virtually free from his useless body. (TOS: "The Menagerie, Part I", "The Menagerie, Part II")
In the mirror universe, the Talosians attempted to trick Terran emperor Philippa Georgiou with their mental projections. In response, Georgiou had their civilization 'blasted from the face of Talos IV'. Georgiou noted this to Leland after Section 31 discovered the power of the Talosians to create illusions. (DIS: "If Memory Serves")
The Talosians were the first aliens encountered on Star Trek, appearing in the earliest Star Trek: The Original Series production, "The Cage". (While Spock was the first non-Human featured, his species is not mentioned in that episode.)
Upon devising the Talosians, Gene Roddenberry reused elements of the species from a story outline he submitted for an ultimately unproduced episode of the television series Science Fiction Theater. The story treatment was titled "The Transporter" and featured the invention of a device – the "transporter" referenced in the title – which, in Roddenberry's words, "creates an artificial world for the user, capable of duplicating delight, sensation, contentment, adventure – all beyond the reach of the ordinary person living the ordinary life." The outline also involved the machine's inventor realizing the device might eventually lead to mankind's destruction or, as Roddenberry put it, "[The machine] might be used as they have used the miracle of radio, television, the motion pictures – with more devastating results…. It could create wants and desires for which the world would destroy itself – a dying race sitting at their 'transporters'." Thus, Roddenberry later made the Talosians capable "of duplicating delight, sensation, contentment, [and] adventure," and established them as a dying race, experiencing life vicariously through others. (These Are the Voyages: TOS Season One, Chapter 1: "The Creator")
The Talosians were originally written as a crab-like species. The story outline for "The Cage" (as reprinted in The Making of Star Trek, pp. 47-65) commonly referred to them as "crab-creatures" and said of the aliens, "Although in no way human, they are obviously intelligent and have digital capabilities via six multiclawed arms and legs." The outline also dictated that, among their own kind, the Talosians were to have used "claw-snap and clatter for speech" and were originally intended to have not only claws but also an "external armor-skeleton" that made similar noises. The aliens were not imagined as being capable of communication, other than the clattering of their claws; the commanding officer of the Enterprise (at that time, known as Captain Robert April) instead understood the aliens by translating their noises via his "telecommunicator" (a device that later developed into both the common communicator and the universal translator). Also, the aliens' mode of moving was referred to as "scuttling." (The Making of Star Trek, pp. 48-49, 58)
Gene Roddenberry discovered crab creatures would be too expensive to build. As a result, the aliens became humanoid. ("The Menagerie, Part I" text commentary, TOS Season 1 DVD) In the second revised final draft script of "The Cage", the Talosians were thus introduced as "small, slim, pale human-like creatures with large elongated heads, suggesting huge and powerful brains. They wear shimmering metallic garb." Roddenberry imagined the aliens as not only thin but also very frail. (The Making of Star Trek, p. 349)
In a line of dialogue which was scripted for "The Cage" but not included in the final version of that episode (nor any other installment), Pike commented, "What's happened to the Talosians could be sort of a warning, couldn't it. For us individually or for a whole race. Our electronic tape, our viewing screens, even our books, must never become a substitute for real life." In another scripted but discarded line, Vina said of the Talosians, "Since their minds can reach anywhere, most of them are like cocoons or larvae now. They just sit and let the thought records or some specimen live for them. Some of them hardly move, except to take that blue protein once a day." This "blue protein" was a reference to the Talosians' protein complex.
The producers and Gene Roddenberry decided to cast the Talosian roles as females and then dub male voices over the footage. In a 1988 interview, Director Robert Butler uncertainly recalled that this idea "might have been" his. He went on to say, "When I saw the characters in the script I thought it would be interesting to get a difference, and one easy difference is to cast women just because of their size and grace, and then add voice-overs later. Therefore you get an oddness, an antisexuality that certainly might be more the case in other galactic cultures than our own, and I think that might have been my notion. But at the same time I remember that when I mentioned it to Gene he had had a similar feeling that we should go bizarre, so there was not much discussion if it was my idea. If I said, 'Hey, let's do that,' he might have said, 'Yeah, I get it, it's a good idea,' or vice versa." (The Star Trek Interview Book, p. 97) The idea of casting women, with their lighter builds, appealed to Roddenberry because he thought it might give the impression that the Talosians had let their bodies atrophy in favor of higher brain development. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD) Thus, Roddenberry searched Hollywood for diminutive actresses who had faces that he deemed to be interesting. (The Star Trek Compendium, 4th ed., p. 15) Once the performers were cast, their breasts were tightly wrapped, in an effort to disguise each actress' female form. (The Making of Star Trek, p. 349)
The design of the Talosians additionally incorporated headpieces that – complete with their bulging veins and small, round ears – were created by craftsman Wah Chang and were blended into the actresses' own facial features by Fred Phillips and his make-up staff. (The Star Trek Compendium, 4th ed., p. 15) Chang's work on the Talosian head prosthetics also included the throbbing quality of veins, but this effect can only be seen on The Keeper's head. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD) Meg Wyllie, the actress who played The Keeper, later remembered the make-up required; "The base was an old-fashioned rubber bathing cap – the type with a chin strap. Above, or rather upon the cap, a rubber substance was placed. When that was set, the cap was removed, placed on a form and the technical special effects people finished the skull – placing the blood vessels and covering them. The makeup was not comfortable – my ears especially suffered being so confined under the bathing cap." (Starlog issue #117, p. 52) To operate the pulsing veins on The Keeper's head, Assistant Director Robert Justman hid out of the camera's line-of-sight and squeezed a small rubber bulb that inflated and deflated the veins. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD) Sandra Gimpel, who also played one of the Talosians, commented, "Well, the makeup took, like, two and a half, three hours. The interesting part about it is how different it was then from now. The heads. The Talosians talk by telepathy, so the veins would pump every time they talked. So they had an air bladder in the head, with the veins. Then a tube ran down my back and down my arm, and I had a ball in my hand. Every time I pushed it, the air would make the bladder move. That's why you'd see the Talosians standing so still. The costumes had really long arms, because they covered up us holding the ball." 
In an unused line of dialogue from the final draft script of "The Menagerie, Part I", James T. Kirk speculated that the Talosians may have developed "some surgical methods" that the Federation had not yet developed, as of the timing of that episode (in 2267). However, Commodore Mendez skeptically replied that the single reference to life on Talos IV in Spock's entire report referred to it as "feeble and parasitic" and that such parasites "hardly make skilled surgeons."
A small action figure of what appears to be a Talosian can be seen on Rain Robinson's desk in the 1996 of the Star Trek: Voyager episodes "Future's End" and "Future's End, Part II". The same or a similar action figure could be found in Production Designer Richard James' office during production on Voyager. (Star Trek: Communicator issue 111, p. 54)
During the making of Star Trek: Enterprise, little consideration was given to bringing back the Talosians, despite other TOS aliens reappearing on the series. Mike Sussman recalled, "To my knowledge, no one on the writing staff was pitching Talosian stories." (User talk:Mdsussman#TOS Aliens)
A Talosian was to cameo in the 2009 film Star Trek. It was redesigned by sculptor Don Lanning. He later referred to it as an "amazing design" and speculated that the aliens "would have been played by women." Lanning went on to explain, "I did a drawing that was pretty much a straightforward make-up, where the actor's real neck would be painted green for digital removal, leaving this little spindly neck sculpted onto the front of the actor, and the body would be worn like a Bun Raku puppet. It was a fully realized make-up that was actually rendered out by Joel [Harlow], and it was a fascinating idea." (Star Trek Magazine Special 2014, p. 137) Harlow crafted the design into a sculpture which featured a paint job by himself and Crist Ballas.  Offered Harlow, "What I did was sculpt the head extra-large with a thin neck on top of his real neck. We just assumed that when they shot it, his own neck would have been green-screened out, leaving a giant head on this tiny little neck. I think it would have looked really cool, but it ended up just being a mask, and all the body stuff, which would have been a rod puppet, was sort of neglected at that point." (Star Trek Magazine Special 2014, p. 137)
Reception and likenesses
Dave Rossi, VFX Line Producer of the remastered version of Star Trek: The Original Series, once enthused about his fondness for the Talosians, "Any aliens that garner the death penalty if you go see them is... They're pretty epic. And it just speaks to their unique power of mind control [....] The Talosians take it [mental powers] to a whole other level. In fact, I would say that, in as far as The Original Series goes, the Talosians are probably one of the most powerful aliens we ever met, and that makes them fun, even though they have fanny heads." ("The Menagerie, Part I" Starfleet Access, TOS Season 1 Blu-ray) With similar gusto, Michael and Denise Okuda commented the casting of females as the otherwise male Talosians was "highly creative." ("The Menagerie, Part II" text commentary, TOS Season 1 DVD)
A similar casting trick was used again much later, with the Sphere-Builders in Star Trek: Enterprise, as they were all women except for one male. The same casting strategy was also used in another of Gene Roddenberry's (posthumous) TV series, Earth: Final Conflict, wherein the Taelons (a species that, coincidentally, bear a striking resemblance to the Talosians) were all played by female actors. The Talosians are similar too, in many ways, to the underground mutants of the Planet of the Apes series. Both are subterranean survivors of nuclear disaster with impressive mental abilities, which include the power to create thoughts and images in the minds of others.
The novel Burning Dreams reveals that Talosians are androgynous and uses the "s/he" and "hir" pronouns to refer to them. According to the novel, Talosian civilization had been revitalized by the year 2320 thanks to the influence of Christopher Pike. An older novel, Legacy, referred to the Keeper using male pronouns.
The Talosians of the mirror universe appeared in the short story "The Greater Good" by Margaret Wander Bonanno, contained in the anthology Shards and Shadows. As in the primary universe, they used a distress call to lure the ISS Enterprise to Talos IV with the intention of having Christopher Pike mate with Vina so as to create a race of Terran slaves. However, Pike rejected her, refusing to mate with an "insipid Human female." Upon learning of the Terran Empire fear of telepaths, the Keeper decided to release Captain Pike and use him as their eyes and ears throughout the empire; the captain would give the Talosians an early warning, should the empire ever decide to attack and obliterate their planet. After assassinating Pike and assuming the captaincy of the Enterprise in 2264, James T. Kirk returned to Talos IV and ordered that its surface be leveled, annihilating the Talosians for the good of the Terran Empire.