Star Trek: Deep Space Nine, often abbreviated to DS9, is the first spinoff series from Star Trek: The Next Generation (and the third live-action Star Trek series) and two characters from that series had major roles in this. The first of these was Chief Miles O’Brien, who was in the series from the beginning as the chief engineer of the titular space station Deep Space 9. O’Brien was joined by Lieutenant Commander Worf at the beginning of season four, after the end of TNG and following the events of the film Generations.
The series is set on a former Cardassian space station, previously known as Terok Nor, which has now come under Federation control. The station orbits the recently freed planet of Bajor, which had been occupied by the Cardassians for many years (this had cropped up a few times during TNG; most notably, Ensign Ro, first seen in the titular episode, was a Bajoran). With it being set on a space station, a type of construction noted for not being terribly mobile (it could move around slightly and did in the two-part opener), this did lead to a description that this series was ‘boldly going nowhere.’ However, the station had a number of Runabouts – small warp-capable ships with a tendency to get destroyed a lot – and later received the first in an experimental class of starship (this ship had a small appearance in the film Star Trek: First Contact).
In the first two-part opening episode, a stable wormhole to the Gamma Quadrant is discovered, opening a new area of the galaxy up, one which has not been explored to date. This wormhole is also the Celestial Temple of the Bajorans’ religion. This makes DS9, and Bajor, much more important than they used to be, as the system is now the way to reach a large chunk of the galaxy, an area that would otherwise take a very long time to reach even at high warp speeds.
Although most of the episodes are standalone to one degree or another, there are various ongoing story arcs. DS9 becomes the focus of various conflicts, with the Cardassians often being involved in one way or another, including a major war, which plays a big role in the series and spans multiple seasons. There are new enemies introduced as well. The Mirror Universe, from the original series episode “Mirror, Mirror”, is visited on a number of occasions throughout the series, and forms another story arc. The wormhole is also inhabited by aliens, who are also the Prophets of the Bajorans’ religion, and these Prophets crop up more than once, as the commander of the station, Benjamin Sisko, becomes their Emissary in the first two episodes. Whether he likes it or not.
There are a number of episodes based around the Ferengi, specifically Quark, the Ferengi bartender and owner of Quark’s in the station, which give an insight into Ferengi culture. Zek, the Grand Nagus and ruler of the Ferengi Alliance, also features prominently in some of these episodes. The Maquis, the resistance against the Cardassians who dislike a treaty the Federation made with that polity, also play a role in another arc.
Quark is played by Armin Shimerman, the first actor to play a Ferengi, albeit a different one, in Star Trek during the TNG episode “The Last Outpost”, and one of the few actors to play the same character in three Star Trek series. Shimerman later appeared in the Warehouse 13 episode “Where and When”, and Rene Auberjonois, who played Quark’s nemesis Odo, had a recurring role in several episodes of the same series as Hugo Miller. Shimerman also appeared in the Stargate SG-1 episode “The Nox”. and Auberjonois in “The Other Side”.
One result of the various different conflicts is that the series features many more space battles than TNG did (that series was mostly free of conflict), and much larger ones at that. Larger, even, than some of the battles against the Borg, probably including that in First Contact.
The USS Voyager is also docked at Deep Space 9 when it is first seen in the first episode of Star Trek: Voyager, “The Caretaker: Part 1”. This probably took place between the DS9 episodes “Past Tense: Part II” and “Life Support”.