It should be noted that the following happened (happens, will happen) in at least one, and more likely several alternate universes, and not in this one (the one in wich we actually live).
In 2365, Jax DeSabel and Sam Lator were on the Enterprise-D when they became acting ensigns, and were sent on a mission to study the Earth's atmosphere in the late 20th century, over a time span of the latter half of the 80's through the early 90's. While on this mission, they met a couple of local kids of about their age, who happened by chance to look rather remarkably like them. Their names were David Ward and Rob McBey. Sam was of course highly skeptical at first of Jax's crazy suggestion that they tell Dave and Rob who they really were. But in the end, Jax convinced Sam it wouldn't screw things up too badly... but only after they'd known them long enough to see they could be trusted with such a secret. (Not that anyone would ever believe them if they told it, anyway.)
So one day, Jax and Sam sat their friends down and told them they were actually from the 24th century, and that they were acting ensigns in the United Federation of Planets' Starfleet, traveled back in time to study the atmosphere as a model for planets in their own time that were having troubles similar to the ones they had had on Earth four centuries ago.
They described the future somewhat more in detail, and Dave and Rob at first thought they were playing some kind of RPG based on Star Trek. Jax and Sam had never heard of Star Trek, but they were interested in hearing about it. It turned out that there was a fictional universe written about in a small cult fanzine that very few people knew about; maybe a million worldwide, suggested Dave. Rob thought that was guessing a little high. Maybe a few hundred thousand. They decided to look into later, but never really got around to it.
This fanzine, called Star Trek, had been created in 1966, and had been temporarily cancelled in 1969. But in the 70's, a few people started producing very low-budget direct-to-video shows based on the old 'zine. By 1987, there was enough renewed interest to get the original fanzine going again, but this time it was set in the 24th century, in a world very like the one described by Jax and Sam. It was around this point that Dave and Rob had first started subscribing, and found back-issues from the first run at the library. They hadn't managed yet to get ahold of any of the videos, though new videos were soon produced set in the new era of the revived fanzine.
On Jax and Sam's next return to their own time (for they did return periodically over the two-year course of their mission), Jax looked into this strange coincidence. Eventually he turned up the fact that, after the original Enterprise had discovered how to time travel, the Federation had started sending small teams of historical researchers into the past for various reasons, and they had sometimes requested a way of getting more regular reports on the status of their home time than they were currently getting. A lieutenant who had served for a time on the Enterprise named Gene Roddenberry came up with an idea. A TV show could be created in the 20th century, which very few people would know was based on reality. Just Gene and a few of the inner circle of writers. The rest who were involved in the show, as well as the fans, would think it was just science fiction. But the research teams could watch the show. It would be set on the Enterprise (a nod on Gene's part to his time on that ship), but it should include enough information to keep the time travelers happy. Of course, sometimes there would be stories produced which were completely fictional, because they would let writers from the 20th century get involved. A little signal would be given that only those in the know would recognize, that would indicate the current story was apocryphal.
However, the network Gene pitched the show to, NBC, didn't go for it. So instead, he just ended up creating a magazine in which stories would be published, which he had hoped to produce for television. By the end of the 60's, research teams were no longer being sent into the past. Gene, however, had come to like the 20th century, and went back there whenever he could (so it would actually seem almost as if he was actually there all the time). Meanwhile, he continued his career in Starfleet. After the success of his Star Trek project, he was promoted to lieutenant commander. Over the decades he rose through the ranks, until finally he was an admiral. When he retired from Starfleet in 2363, he moved to the 20th century permanently. Still, he wanted information on his home time, so the magazine was revived. It was a stroke of luck that the original had in the intervening years of the century achieved a cult following, which would allow a simple explanation for the re-emergence of the fanzine.
Jax managed to get ahold of the videos up to the point in time he would be returning to in the 20th century, and gave them to Dave and Rob, who themselves managed to locate a distributor of such videos for the future. The friendship between the four continued to grow, and even after Jax and Sam returned to their own time when the mission was finally complete, Jax and Dave kept in touch. Sam and Rob occasionally exchanged correspondence themselves through the other two.
Before the mission, Jax and Sam had both been working on possible ways of getting into the past without having to slingshot a whole ship around a sun everytime, especially considering there wouldn't be a warp-capable starship in the 20th century to use for their return voyage, as the one to get there would have to return home after dropping them off. Sam was working on a quantum leap project, but got sidetracked by a discovery in the course of his research which would later lead to the development of quantum torpedoes, by a team of Starfleet scientists following his early data. Jax, meanwhile, had his own ideas, which he worked on with Geordi La Forge. They developed a photon torpedo casing which contained a transporter matrix and communications relay, instead of explosives. A special launcher was also developed which could fire the torpedo at warp speed with a preprogrammed course around a sun. A few such launchers were developed, one of them for the Enterprise. Before launching the torpedo, Jax and Sam and anything they might want to bring with them would be dematerialized and stored in the transporter matrix in the torpedo, which would be programmed to rematerialize them after landing safely and out of sight in a rurual location on Earth in the 20th century.
After the mission, Jax gave Dave a few parting gifts, including a torpedo launcher. He also called in a favor from a friend who worked at Space Control in orbit of Earth in the 24th century, and gave him a torpedo launcher. He would act as a middle man between Jax and Dave, receiving and sending transports between the two. Often these would just be messages recorded in the torpedo's communications device. Sometimes Dave would send Jax recordings of TV shows, movies, or CDs from his own time, since they had similar tastes in entertainment. (This didn't cost Dave anything much really, since another of Jax's gifts had been a modified replicator that could produce at most two copies of a thing, but nothing too expensive, because he didn't want to mess up the economy of the time.) Jax could have found records of such entertainments in his own time, but it was far simpler just to let Dave do it than spend time looking through the records and trying to decide what sounded promising. Jax also sent recordings of such things from his own time and earlier times, though later than David's time, to Dave, from Earth and from other planets. And they kept each other informed of developments in their respective times, and just friendly messages about their own lives.
Eventually, Dave started writing in Star Trek rounders on the Internet, which were jointly written by fans of the fanzine and/or videos. When he told Jax about this, Jax thought it'd be fun to keep up with what they all wrote, and perhaps provide Dave with little pieces of the truth about the future to include in some of his posts in the story, although most of what Dave wrote he'd make up himself. Even if much of it was based on his knowledge of what Jax was like, and some of the people Jax talked about sometimes...