(covers information from several alternate timelines)
The Nexus was an extra-dimensional realm in which one's thoughts and desires shaped reality. Inside the Nexus, time and space had no meaning, allowing one to visit any time and any place that one could imagine. The doorway to the Nexus was a violent, destructive temporal energy ribbon which crossed through the galaxy every 39.1 years, among others in 2293, 2332, and 2371.
Starfleet first encountered this phenomenon in 2293, when the newly-christened USS Enterprise-B responded to a distress call from two vessels transporting El-Aurian refugees to Earth. Arriving at the vessels' location, the Enterprise found them trapped in a powerful gravimetric field emanating from the trailing edge of the energy ribbon.
Arriving too late to save the first ship, the SS Robert Fox, the Enterprise moved into transporter range in an attempt to beam aboard the crew and passengers from the other, the SS Lakul. The crew experienced difficulty in maintaining a transporter lock on them, however, as their life signs were experiencing a state of temporal flux, phasing in and out of the spacetime continuum. As the Lakul began to lose structural integrity, the transporter was initiated, but the crew was only able to save 47 people out of 150, including El-Aurians Guinan and Tolian Soran.
Afterward, the Enterprise itself became caught in the gravimetric field, and was unable to escape. Montgomery Scott, who was attending the maiden voyage of the vessel along with Captain Kirk and Commander Chekov, realized that a resonance burst fired from the main deflector might disrupt the field long enough for the ship to break free. Although this was successful, Captain Kirk himself was pulled into the Nexus when an energy discharge from the ribbon impacted the section of the ship where Kirk was modifying the deflector relays. For the next seventy-eight years it was believed that Kirk had died a hero. (Star Trek Generations)
Soran's attempt to return to the Nexus
Before being rescued by the Enterprise, the El-Aurians experienced brief moments inside the Nexus, where all of their desires became reality. Soran became obsessed with returning, and dedicated himself to discovering a way to safely re-enter the Nexus, rather than the more random possibility of flying into it in a ship in the hope that he would be taken into the Nexus before his ship was destroyed.
By 2371, he had developed a plan to destroy two stars. First, he came to Amargosa system to launched the solar probe from the observatory to destroy the Amargosa star altering the gravitational forces influencing the course of the ribbon and bringing it to the planet Veridian III, where Soran would be waiting for it. Captain Jean-Luc Picard of the USS Enterprise-D discovered the nature of Soran's plot, and realized that, by using the solar probe destroy the Veridian star, he would also destroy all of the planets in the Veridian system, including Veridian IV, a planet with a pre-industrial population of 230 million. Picard confronted Soran on Veridian III in an attempt to stop him, but was unsuccessful in preventing Soran from destroying the star. As all planets in the Veridian system were destroyed, as was the Enterprise-D's saucer section, both Picard and Soran were pulled into the Nexus.
While inside, Picard encountered Guinan, who described herself as an "echo" of the version of Guinan who once visited the Nexus, allowing him to realize what had happened to him despite experiencing his perfect world. With Guinan's advice reminding him of his responsibilities outside the Nexus, Picard located Kirk, who, from his point of view, had only just entered the Nexus after his loss on the Enterprise-B.
Picard initially had difficulty in persuading Kirk to leave the Nexus, as Kirk saw it as his opportunity to correct the mistakes of his past. Kirk soon realized, however, that the events which transpired in the Nexus were not real, but rather a complicated illusion, and agreed to join Picard, recognizing that the fundamental unreality of the Nexus meant that the one thing it could never offer him was the one thing he had always wanted; the chance to make a difference. Due to the timeless nature of the Nexus, the two captains were able to leave and arrive on Veridian III at a point in time before Soran destroyed the star. Teaming up to fight him, they were successful in changing history and stopping Soran, but unfortunately Kirk was killed in the process. (Star Trek Generations)
In the Nexus
When speaking about the Nexus to Captain Picard, Guinan described it as like "being inside joy." Inside the Nexus, all of a person's desires become reality, and one can reshape that reality to visit any time and any place one chooses.
Picard's version of the Nexus was of a family he never had. He had two sons – Matthew and Thomas – and three daughters – Olivia, Mimi, and Madison. They lived, with his wife Elise, in a beautiful house, and his nephew René, who had recently died in a fire in the real world, was still alive.
Kirk's version was initially being at his log cabin in the mountains, which he had previously sold. While there, he reunited with his dog Butler, and had the chance to propose marriage to Antonia, a woman whom he had regretted leaving to return to Starfleet. Later, he experienced his uncle's farm in Idaho, where he went horseback riding with Captain Picard. (Star Trek Generations)
In an early version of the Star Trek: Generations script, Guinan explains that her experience with the Nexus and the Enterprise-B are responsible for her "sixth sense" that lets her perceive people and events outside of linear time.
Ronald D. Moore commented:
- "Guinan, Soran, and the other El-Aurians are in some kind of transitional phase going into the Nexus when their ships begin exploding. The Enterprise-B beams them away at a crucial moment that brings their physical bodies back, but leaves behind some kind of "echo" (at least for Guinan – whether or not Soren also left behind an "echo" was the subject of much debate and many rewrites, some including a Soren doppelganger and some even including a meeting between the two Sorens in the Nexus). The momentary sensation of being in the Nexus leaves both Guinan and Soren with an almost overpowering desire to return. When Kirk is pulled into the Nexus, there's no transporter beam to yank him away and hence, he's completely left inside. When Picard and Soren enter the Nexus years later, they too are pulled in completely and the idea of an "echo" should not apply since it was the direct result of a transporter fluke in a very specific instance." (AOL chat, 1997)
The novelization of Star Trek Generations expands on Kirk's fantasies in the Nexus, with its timeless nature allowing him to experience multiple events while still feeling like no actual time has passed. After Picard enters the Nexus, the novel depicts one fantasy where Kirk is preparing for his wedding to Carol Marcus with all his friends and colleagues from the Enterprise – with Leonard McCoy expressing shock at the idea of Spock having brought a date and David Marcus giving his mother away at the altar – as well as referencing other encounters with other women from his past, including an occasion where he managed to save Edith Keeler and preserve the future.
In the novel Engines of Destiny, a confrontation between an alternate Guinan and the Guardian of Forever reveals that the version of Guinan in the Nexus is the source of her constant "feelings," as this version of herself exists outside all time, and can thus reach out to her other selves and provide valuable insight into possible actions that must be taken, although even she is ignorant of what the definitive outcome will be in the end.
During the novel, Scott travels back in time in a Klingon Bird-of-Prey with the intention of beaming Kirk out of the Enterprise-B before he falls into the Nexus – believing that this will ensure Kirk's survival without actually changing history – only for history to change so that the Borg conquer the Alpha Quadrant (the reason for this chain of events is never explicitly revealed to the characters, but the evidence provided to the reader suggests that this is due to the Borg's invasion of the past in Star Trek: First Contact succeeding as Picard apparently failed to survive his confrontation with Soran without Kirk's aid). At the novel's conclusion, history is restored when the Enterprise-D – which had followed Scott into the past and thus been protected from the changes in history – manages to send Kirk back into the Nexus in the new timeline, undoing all subsequent events.
In The Next Generation - Q Continuum, it is revealed that the Nexus was created from a solar flare by a very much younger Q during an idle moment in his "childhood," but he was ignorant of what had become of it.
In the Star Trek: Stargazer novel Oblivion, chronicling Guinan's "first" meeting with Picard (discounting Picard's meeting with Guinan's past self in "Time's Arrow"), it reveals that, after her initial encounter with the Nexus, Guinan was left in a deep state of depression, mourning the loss of the reality she had in the Nexus, where all the family she lost to the Borg – including her youngest daughter, the one who was most like her – were restored to her. She experiences a particularly bleak mood after the Nexus passes through the galaxy once again, but after meeting with Picard, who subsequently risks his mission to save her, Guinan overcomes her depression, realizing that the universe can still give her joy and friendship even outside the Nexus.
The Nexus makes an appearance in the Star Trek Online mission "Beyond the Nexus", as part of the celebration of the 30th anniversary of Star Trek: The Next Generation. When the Nexus makes its passage again in 2410, 39.1 years after the events in Star Trek Generations, Starfleet receives distress call from the USS Forrestal, an Ambassador-class vessel that disappeared in the Nexus during its passing in 2332. The player character is teamed with Captain Geordi La Forge and the USS Challenger to discover that the Forrestal and the Galaxy-class USS Madison, which had been sent to investigate, have been taken over by a psionic entity called Khaj'Buur, who was imprisoned in the Nexus. It is mentioned in the mission's conclusion that Khaj'Buur's desire to leave the Nexus is highly unusual, considering its paradise-like nature.