Star Trek: Deep Space Nine is the second of the "next generation" of Star Trek shows, airing after The Next Generation for two years, then alternating with UPN's Voyager for the remaining five years. Set on an orbital space station, DS9 traded the Wagon Train to the Stars premise for "Fort Apache in Space". Ira Behr, the head writer, cited The Rifleman as another influence (particularly the father-son dynamic).
When we last left Next Gen, the remote world of Bajor had just booted out its occupiers through a war of attrition and a fair amount of terrorism. With the planet spiraling into anarchy, Starfleet sent a detachment of officers lead by Commander Sisko as a diplomatic liaison and provide aid to the Bajorans. They take up residence at a Cardassian station called Terok Nor, rechristened Deep Space 9 as a makeshift Starfleet outpost. In the pilot, a one-of-a-kind stable wormhole is discovered, leading to a distant and uncharted section of the galaxy. Instantly, Bajor is transformed from a rustic backwater into the most valuable piece of real estate in the quadrant, and DS9 is assigned to monitor the wormhole's traffic. The appearance of the wormhole is seen by the Bajorans as fulfilling a religious prophecy and Sisko is declared to be "The Emissary." The fixed base allowed the show to delve deeply into the politics of the established Star Trek universe, but the appearance of the wormhole also caught the attention of the Dominion, a less cuddly counterpart to the United Federation of Planets.
What made DS9 so unusual in Trekdom was that every action had consequences. Part of it is the writers becoming more comfortable altering Gene Roddenberry's spotless, optimistic future: nobody on Bajor particularly got along with each other and, unlike the ship-based series, the crew couldn't just 'jump to warp' and leave the Problem of the Week behind. The writers employed Story Arcs much more extensively than in other Treks (including a full Myth Arc dealing with the Dominion), showing it had now earned the "Space Opera" genre tag that it had been given. Perhaps most importantly, by shifting focus from TNG's bold explorers to DS9's overworked jarheads, the writers were able to scrutinize the Federation as it appears to outsiders: a noble organization which still has problems with bureaucracy and some skeletons in its closet, but is also still a symbol of beliefs that many of the main cast believe in fiercely.
While all Trek shows have large casts, DS9 is the only one which qualifies for Loads and Loads of Characters, by virtue of having a fixed location and a population surrounding it. Consequently, the show was overrun with Fake Guest Stars, Aron Eisenberg (Nog), Max Grodenchik (Rom) and Andrew Robinson (Garak) standing out in particular; all three characters appeared within the show's first two episodes and were heavily featured straight through to the series finale. This was a show that could do a Bottle Episode but have it star people who weren't even in the opening credits; one focused on Nog's recovery from a war injury with the help of a second recurring guest, and another on the disappearance of an Ascended Extra who never once had a line!
As a result of this kind of thing, the show tends to divide Trekkies quite a bit: people who like Trek for the morality plays and spacefaring action may be turned off by the soapy melodrama and deconstruction of Roddenberry's utopian theme. On the other hand, those who do like DS9 tend to (heavily) prefer it over other Trek shows, forming a little subculture of their own in Trekkie fandom known as "Niners".
Deep Space Nine is very frequently compared to Babylon 5, the other 90's cult Space Opera show set on a space station, incorporating matters of faith, focusing on conflicts between interstellar empires, having Loads and Loads of Characters (aided, in B5's case, by a lot of Cast Turnover), featuring strong Myth Arcs, and deconstructing the future. Indeed, there were frequent accusations that the two shows had ripped one another off; J. Michael Straczynski has implied that Paramount effectively stole an idea that he pitched to them and stuck a Star Trek brand on it. Although DS9's and B5's pilots are similar, Season Four was the point at which the two began to dovetail.
The show currently runs in... British and Japanese TV. It used to run in Syndication on Spike TV in the United States, but due to low ratings has not aired for some time. As of October 2011 the complete series is available on Netflix streaming in the United States (where, by many reports, it has been far more popular than it was in rerun broadcast and is a staple of the service). For that matter, it is also on Amazon Instant Video, Hulu and CBS All Access as well, so basically you're covered if you're signed up on any of these. Alongside all the other Trek shows and films, DS9 was made available in its entirety in Netflix Europe in 2016.
In 2017, Ira Steven Behr and Adam Nimoy announced they were crowdfunding a documentary on the series, What We Left Behind: Looking Back at Star Trek: Deep Space Nine. In addition to bringing back most of the major actors, the film is to look at DS9's cultural impact, particularly its post-wrap rise in popularity in syndication and streaming, and will feature Behr getting his writers' room together to plot a "what if" season 8.
Related works in the Star Trek Expanded Universe include the Terok Nor trilogy, a sub-series of Star Trek: The Lost Era which chronicles the Occupation of Bajor and features many of the Bajoran and Cardassian characters, and the Star Trek: Deep Space Nine Relaunch continuation series.
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