(covers information from several alternate timelines)
The Guardian of Forever was a mysterious construct of an unknown, ancient alien race, which functioned as a time portal; a gateway to the time vortex that allowed access to other times, locations and dimensions. It was located on an ancient planet where the focus of all timelines throughout at least the Milky Way Galaxy converged. It was found among the ruins of a large, forgotten city that stretched beyond the horizon in all directions around it. Based on initial observations, the ruins appeared to be at least one million years old. The Guardian said that it was far older than that, even exceeding the Earth's sun's age of 4.6 billion years.
An inert, crudely asymmetrical, circular gateway comprised of a quasi-metallic substance, the Guardian was able to somehow display the flow of time itself although it could not alter the "speed at which it displayed the past." It generated immense ripples in time that manifested themselves as spatial disturbances in the region around the planet where it was located. Apparently sentient, it was capable of speech and interacted with anyone it encountered, responding to external stimuli such as questions and actions. The object would flash whenever it spoke and emitted a gaseous fog within its center whenever it generated a portal.
Contact with the Federation
The Guardian was first discovered by the crew of the USS Enterprise in 2267. Encountering powerful waves of space displacement, which Spock described as "ripples in time", the Enterprise tracked the waves back to their point of origin on a previously-uncharted planet. Doctor McCoy, suffering from paranoid delusions as a result of an accidental overdose of cordrazine, beamed down to the surface in an attempt to escape the ship.
Searching for McCoy, Captain Kirk and Spock encountered and made contact with the Guardian, who offered them the chance to explore the past. As the Guardian was displaying images from Earth history, McCoy emerged from hiding and leapt through the time portal, arriving on Earth in the year 1930.
The landing party soon discovered that they had lost all contact with the Enterprise, and the Guardian informed them that McCoy had effected a change in history, wiping out their civilization. Realizing that they must correct the damage to history, Kirk and Spock had the Guardian replay Earth history, and traveled through the portal to a point in time prior to McCoy's arrival. Eventually successful in their effort to restore the timeline, the Guardian returned all three of them to their proper place and time only a moment after they initially departed. (TOS: "The City on the Edge of Forever")
In 2269, a team of historians, accompanied by Captain Kirk and Spock, used the Guardian to investigate Federation history. Their investigation included firsthand accounts of the formation of the Orion civilization and the monitoring of Vulcan history of the 2230s and 2240s. While Kirk and Spock were visiting Orion, their support team was monitoring Vulcan's past, which, in doing so, inadvertently removed Spock from the proper timeline.
Spock, however, was protected from the change while he was in Orion's past, and the change to the timeline went unnoticed until he and Kirk returned through the Guardian. Once the cause was determined, Spock was able to use the Guardian to return to his own childhood on Vulcan, and prevent his death during the kahs-wan ritual. (TAS: "Yesteryear")
Images seen through the Guardian of Forever
In the original teleplay for "The City on the Edge of Forever", the Guardians of the Time Vortex were 9-foot (2.7 meter) tall, humanoid statue-like beings.
The original Guardian was designed by Desilu Supervising Art Director Rolland M. Brooks, because his colleague, regular Set Designer Matt Jefferies, was sick with the flu that week. (Star Trek: The Animated Series DVDs special feature ""Yesteryear" text commentary") When Jefferies returned to his duties and saw the lopsided donut-shaped set piece for the first time, he reportedly exclaimed, "What the hell is this?!" according to Story Editor DC Fontana. (These Are the Voyages: TOS Season One, 1st ed, p. 517) Special Effects Artist Jim Rugg was responsible for the light effects for the Guardian. (Cinefantastique, Vol. 17, No. 2, p. 24) The voice of the Guardian was performed by Bart LaRue in "The City on the Edge of Forever".
The footage seen in "The City on the Edge of Forever" through the time portal is, for the most part, lifted from old Paramount Pictures and RKO Pictures films, such as the 1945 swashbuckling film The Spanish Main in the latter case. It should be noted though, that Desilu had not yet been acquired by Paramount's holding company Gulf+Western at the time, and that, while much of RKO's assets were acquired by Desilu Studios after the former went defunct in 1957, its backlog catalog of movie productions was not part of it, it being divided over several other major Hollywood studios. The use by Desilu of these clippings in the episode therefore, regardless of their origins, essentially constituted copyright infringements in current understanding.
The Guardian's last words in "The City on the Edge of Forever" were: "Many such journeys are possible. Let me be your gateway." Although this suggests that further stories could have been written involving the Guardian of Forever, the Guardian's only other on-screen canonical role was in the animated episode "Yesteryear". In that installment, its voice was performed by James Doohan.
An original draft of the episode that eventually became TNG: "Yesterday's Enterprise" featured a Vulcan science team researching history through the Guardian of Forever. In that story, the team accidentally caused the death of Surak, the father of modern Vulcan philosophy – as a result, the Time of Awakening never occurred, and the Vulcan race had essentially evolved as the Romulans. The timeline was reset when Sarek, aboard the Enterprise-D to greet the scientists, used the Guardian to go back in time and take Surak's place in history. (Star Trek: The Next Generation Companion)
In The Worlds of the Federation, the Guardian's planet is designated as Time Planet.
The Guardian appears in the game Star Trek Online in a mission involving the players character breaking the quarantine of the guardian's planet. The player must travel through the portal in order to follow a group of Klingons and stop them from destroying the Enterprise from the Original series and rescue Miral Paris, who they kidnapped to create and distribute a cure to the Klingon augment virus. Also, if you question the Guardian about why it speaks in riddles, it gives the same response that it did to Spock: that it answers as simply as their limited understanding allows.
In the novel Yesterday's Son, Kirk and Spock use the Guardian to rescue Spock's son Zar from the ancient, doomed world of Sarpeidon. They also discover that Zar is able to communicate telepathically with the Guardian. In the novel's sequel Time for Yesterday, the Guardian is summoned away from its duties of regulating time in the Milky Way by its capricious Creators. Zar uses his telepathic powers to return the consciousness of the Guardian to the gateway and banish the Creators to a universe where they cannot harm our space-time.
In the novel Engines of Destiny, the Guardian appears in an alternate timeline where the Borg have conquered Earth; having learnt of the change from her 'prime' counterpart, an alternate version of Guinan visits the Guardian's planet to ask for help, the Guardian revealing how the timeline can be restored to normal.
In the novel The Devil's Heart, the titular object, an ancient stone rumored to have vast powers, is revealed to be a "seed" created by the same civilization who created the Guardian, and meant to create another Guardian on the world it was sent to, before it went astray.
In the novel First Frontier, members of the Clan Ru, a species whose pre-sentient ancestors – Earth dinosaurs – had been rescued by the Preservers before the mass extinction, used the Guardian to destroy the asteroid which would hit Earth, allowing their species to remain and evolve there. The timeline would be reset by Kirk, with the assent of the Clan Ru, when they discovered that in this altered history, their species destroyed itself in a series of nuclear wars before it could reach the stars.
In the Star Trek: The Next Generation - Q Continuum series, during a flashback to several thousand years ago, the Guardian is used by a younger Q when attempting to find something new, allowing him to make contact with the being known as 0 (although the Guardian briefly tries to deny 0 access to this universe), who subsequently contacts (*), Gorgan and The One via the Guardian. It is hinted here that the Guardian was built by the race that would eventually evolve into the Q – when looking at the Guardian, the young Q comments "At least our ancestors made things", reflecting his dejection at the stagnant nature of the Q Continuum. Q states that a building on the Guardian's planet was built by one of his direct biological ancestors.
In the alternate future seen in the Star Trek: Deep Space Nine book trilogy Millennium the Guardian of Forever was key to Admiral Kathryn Janeway's Project Forever. Janeway, along with a combined Federation/Borg armada hoped to use the Guardian to go back in time and wipe out Bajor (the Federation was at war with the Bajoran Ascendancy in this timeline and were facing the end of the universe). However, at the very moment that Janeway and a team arrived at the Guardian, the Grigari set off a singularity bomb, creating a black hole that destroyed the entire Federation/Borg fleet, the Grigari fleet, Janeway, and the Guardian. The timeline was later reset by Benjamin Sisko and the crew of Deep Space 9.
In the alternate future presented in Imzadi, set in 2406, Admiral Riker used the Guardian to travel back in time to save Deanna Troi's life from an attempt to kill her and prevent her participation in a conference in 2366. In the timeline where she died, the species involved went on to secretly rebuild and become a major military power, but with her survival, Troi's empathic powers revealed that they were lying to gain time, and the conference was abandoned until the species was in desperate need of assistance and were subsequently permitted to rebuild under strict observation.
Although the future Data also attempts to travel back in time to maintain continuity (Riker decides to travel back after a new autopsy reveals that Troi was killed by a poison that was only developed after her death, suggesting that she was killed by a time-traveler, but Data feels that Riker is clutching at straws as the poison in her system could have been a natural fluke), it is revealed at the conclusion of the novel that history had already been changed, and Riker's actions actually set it back on the right path, with the travelers incredulous that the Guardian never told them that detail simply because they never asked.
In Spock Vs. Q, Spock mentions having used the Guardian to travel back to the 20th century. Q describes it as "that lop-sided donut thing" before Spock corrects him, and asks him if he knows it. Q responds "been there, done that, got the T-shirt."
In the novel Provenance of Shadows, the Enterprise returns to the Guardian for a third time, just before the end of its original 5-year mission. They respond to a distress call from the science station erected in orbit of the Guardian's planet. Three Klingon ships attack the station and the Enterprise. They are successful in boarding the Enterprise's bridge and manage to take over the ship, forcing the crew to abandon ship, but Captain Kirk manages to escape, obtain several phasers, and transport down to the planet. There, he steps through the Guardian and ends up back on the Enterprise's bridge moments before the Klingons boarded. As the crew evacuates the bridge, Kirk sets the phasers on overload, and sacrifices himself to kill the Klingons, but his now-alternate timeline counterpart survives. As a last resort, the Klingons crash their ship into the Guardian, apparently destroying it.
In the short story "Guardians" (from the anthology Strange New Worlds VII), the Horta are assigned by the Federation to serve as the protectors of the Guardian after a failed attack/attempt to alter the timeline by the Romulans. Since their lifespan extends for the next 40,000 years, they are witnesses to vast changes in the Federation, the ascendancy of Humanity to incorporeal beings and the eventual exploration of the known universe and beyond. The Guardian undergoes periodic attacks and attempted usage from the Borg, the Romulans and the Crystalline Entity, each time defeated by the Horta.
The Guardian is also featured in the novel The Star to Every Wandering, in which it plays a sizeable role. In this novel, the events of the film Star Trek Generations play out differently, with Kirk's emergence from the Nexus resulting in a temporal loop with catastrophic effects. This Kirk, returned to the Nexus by these events, then recruits another version of himself from within the Nexus, who then rescues yet another Kirk from the USS Enterprise (NCC-1701-B) by employing the Guardian of Forever to travel through time. This third Kirk then travels to Veridian III and dies helping Picard save the Solar System, while the second Kirk ends up contemplating using the Guardian to travel back in time and save Edith Keeler's life without damaging history.
- The City on the Edge of Forever, the original story outline and screenplay published by Harlan Ellison
- The Nexus: a formless, infinite plane of existence shaped by its inhabitants' individual desires and dreams; where the rules of the physical universe, such as that of time and space, don't apply. Should anyone wish to leave, they could literally go to any point in the spacetime continuum of their choosing. (Star Trek Generations)