Guardian of Forever

The Guardian of Forever was a portal-like device used for viewing the past and for travelling through time. It was constructed approximately six billion years ago by an unknown race on the planet Gateway.

History of the Guardian
Two million years ago, a young Q encountered the Guardian, which resulted in his meeting the malevolent extra-dimensional entity known as 0. One million years ago, 0 used the Guardian to bring other entities like itself into the Universe (TNG novels Q-Space and Q-Zone)

The Federation's first contact with the Guardian of Forever was made in 2267 by Captain USS Enterprise. Doctor Leonard H. McCoy, suffering from cordrazine-induced madness, fled through the Guardian to 1930 Earth. Kirk and Spock followed him and succeeded in preventing him from altering history, but at the cost of the life of Edith Keeler, with whom Kirk had fallen in love. The incident made a profound emotional impact on Kirk. (TOS episode: "The City on the Edge of Forever")

In 2269, Spock assisted a team of historians on Gateway, only to find that history had been altered so that he had died as a child. He used the Guardian of Forever to travel to 2237 and save the life of his younger self (TAS episode: "Yesteryear")

Later that year, Spock gained permission to use the Guardian of Forever to visit Sarpeidon's past and bring his son Zar home with him. However, circumstances necessitated Zar returning to his own place in space and time. (TOS novel: Yesterday's Son)

Shortly after this, when historian William Harrod "accidentally" fell into the Guardian of Forever, Uhura was sent to retrieve him. (TOS short story: "If I Lose Thee...")

Towards the end of the year, Kirk and the Enterprise crew used the Guardian of Forever to prevent the Clan Ru from altering Earth's history (TOS novel: First Frontier)

In 2285, Kirk, Spock and McCoy were sent by Starfleet Command to repair the malfunctioning Guardian. (TOS novel: Time for Yesterday) It was shortly after this that the Federation set up the Ellison Research Outpost on Gateway to study the Guardian and the timeline. (novel: Federation)

In 2293, Kirk again visited the Guardian. (novel: Federation)

Roughly around 2364, Dr. Elias Frobisher read his own obituary on a playback of the Guardian of Forever. (TNG novel: Double or Nothing)

In 2368, Captain USS Enterprise consulted the Guardian of Forever regarding the Devil's Heart. (TNG novel: The Devil's Heart)

In 2373, Special Agents Dulmer and Lucsly of the Federation Department of Temporal Investigations used the Guardian of Forever to visit Earth in 2063 and 1996 to follow up on temporal incursions by the USS Enterprise and the USS Voyager. (VOY short story: "Almost... But Not Quite")

During the Dominion War, around 2374, Roga Danar was recruited by Section 31 to prevent the Dominion from seizing the Guardian. After completing his mission, Roga asked the Guardian what it desired. When it said that it wanted freedom, Roga granted it permission to leave Gateways, which it did (short story: "Orphans")

The Guardian in alternate timelines

 * In one alternate timeline, in which Kirk and Spock fail in their mission to prevent McCoy from saving Keeler's life, Chief Engineer Montgomery Scott and Security Officer Michael Jameson travel to 1930 Earth from 2267. Instead, Kirk, distracted by Scott, is killed in the auto accident. This creates another alternate timeline in which Lt. Uhura and Security Officer Worsley travel back in time and join the other four officers to successfully restory history (TOS short story: "Triptych")


 * In another, Kirk (circa 2269) was abducted by Kor, subjected to a Klingon mind sifter, and transported through the Guardian of Forever to 1950s Earth, where he was placed in a mental hospital. Spock was promoted to Captain and spent a year looking for Kirk. (TOS short story: "Mind-Sifter")


 * In another, in which the 2063 Borg invasion of Earth was successful, the Guardian of Forever was consulted in 2293 on how to restore the timeline. (TNG novel: Engines of Destiny)


 * In another, Admiral William T. Riker used the Guardian of Forever in 2408 to travel to 2368 and prevent the murder of Deanna Troi by a time-travelling Sindareen. (TNG novel: Imzadi)


 * In another, the Horta become guardians to the Guardian of Forever in 2464. (TOS short story: "Guardians")