(covers information from several alternate timelines)
The Nexus was an extradimensional realm in which one's thoughts and desires shape reality. Inside the Nexus, time has no meaning, allowing one to visit any time and any place that one can imagine. The doorway to the Nexus was a violent temporal energy ribbon which crossed through the galaxy every 39.1 years, amongst others in 2293, 2332, 2371 and 2410.
Starfleet first encountered this phenomenon in 2293, when the newly-christened USS Enterprise NCC-1701-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 lifesigns 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. Scotty, who was attending the maiden voyage of the vessel along with Captain Kirk and Commander Pavel 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 re-enter the Nexus. By 2371, he had developed a plan to destroy two stars, altering the gravitational forces influencing 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 NCC-1701-D discovered the nature of Soran's plot, and realized that, by destroying the Veridian star, he would also destroy all of the planets in the Veridian system, including Veridian IV, a planet with a 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, 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. With her help, Picard located Captain Kirk, who, from his point of view, had only just entered the Nexus. 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 Captain Kirk was killed in the process. (Star Trek Generations)
In the Nexus
Describing 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, 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)
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 novelisation of Star Trek: Generations expands on Kirk's fantasies in the Nexus, depicting 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-, as well as 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, Scotty 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, apparently as a result of their invasion to 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 Scotty into the past and thus been protected from the changes in history – manages to send Kirk back into the Nexus.
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. However, 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.