The X-Files, FOX
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Caution: potential spoilers.
One of the best shows of all time (or at least I thought so at the time). It was, IMO, the definitive series of the 90's. (It ran for nine seasons, from 1993-2002. I'm fairly sure I didn't have FOX until 1995, so it's quite possible I missed the first two seasons. I really don't remember; I might have found some other way to watch it. Or not.) My review could just as easily go under various categories, such as "science fiction" or "mysterious" or "weird," but I think generally speaking, "supernatural & paranormal" tends to be the best fit. Um... the show alternated between "monster of the week" stories (which could be about actual monsters, or aliens, or mutants, or psychos, or whatever) and "mythology" stories. (This was the first show to use the term "mythology" to refer to its complex, interconnected, over-arching mysteries and conspiracies. The "mythology" or "myth arc" concept has been used by any number of other shows, since then.) Anyway, throughout most of the series, the main characters were Fox Mulder and Dana Scully, who were FBI special agents. (The stars, David Duchovny and Gillian Anderson, acquired reputations as, like, "the thinking person's" sex symbols.) Mulder worked in a division called the X-Files, and investigated all sorts of paranormal stuff, including aliens and whatnot. Scully was a skeptic, a doctor (pathologist), and a lapsed Catholic. There were people who didn't like some of the stuff Mulder was up to, so they teamed Scully with him, hoping she'd gather evidence to debunk his work. Oh, and I should mention Mulder and Scully's boss was Assistant Director Walter Skinner (Mitch Pileggi). He was more or less on their side, though it wasn't always easy for him to be. So he wasn't always particularly happy with them (especially not Mulder).
But Scully didn't find any evidence against Mulder. Over time, she came to realize most if not all of the out-there stuff he believed was actually true. Some of the show's mysteries involved Mulder's family, going back to like Roswell or something. His father had been involved with the Syndicate (the major conspiracy group of the series), I believe, and when they were kids, Mulder's sister had been abducted, and never returned. Samantha's abduction was a large part of what drove him to seek the truth. Anyway, all kinds of strange things happened with the mythology over the years, a great deal of it related to Cigarette-Smoking Man (William B. Davis), who was the member of the Syndicate most familiar to the audience. (He was known in the fandom by various names, such as "Cancer Man" or just "the Smoking Man," though I recall often seeing him referred to as "CSM," so I always go with "Cigarette-Smoking Man." We did eventually learn his real name, though. Which I won't reveal.)
Anyway, eventually Mulder disappeared (around the end of season 7, I think), and agent John Doggett (Robert Patrick) came in to head up the search for him. And a little later another agent, Monica Reyes (Annabeth Gish), got involved. They were both assigned to the X-Files, and were the main characters in seasons 8 and 9, I believe. Of this pair, Reyes was the believer and Doggett the skeptic. Anyway, Mulder was eventually found. And soon after had to go into hiding, for almost the entire final season.
Over the years, we learned a hell of a lot of secrets of the conspiracy, but I don't think we could ever know everything, really. (Perhaps the greatest mystery was the question of where the hell Mulder and Scully got their awesome flashlights. I'd still like to learn the answer to that one.) But there was so damn much we did learn that it's practically impossible to keep it all straight or remember it all. And as is the common consensus among fans, the quality of the show declined in the last few seasons, especially without Mulder around. And even the best of the series... well, I find myself a bit less interested, in retrospect. The 90s are over. Still, it was pretty good. Very interesting, creepy, noirish, with more humor than you might expect (but no less dry).
I also need to mention that there have been two theatrical movies so far, Fight the Future (in 1998, while the show was still ongoing), and I Want to Believe (in 2008, six years after the show ended). Also, there was a short-lived spin-off series called The Lone Gunmen (in 2001), about a trio of conspiracy theorists who had occasionally worked with Mulder.
I also want to mention that I have a sort-of soundtrack called Songs in the Key of X, which has the theme music (which is nicely eerie and a perfect fit for the series), as well as a bunch of cool but dark songs by different artists, none of which I'm aware of having actually been used in the show. But it's one of my favorite CDs. Speaking of music, a song which isn't on the CD, and definitely has no real connection to the series, is Mulder and Scully, by the band Catatonia. And there's a song called David Duchovny, by Bree Sharp. I'd definitely recommend checking out both of those songs, if you're unfamiliar with them. And the show has inspired so many other things, it's just... a cultural icon.
Update: In 2016, there was a six-episode "tenth season," which I initially listed separately under "limited series," but now that it's over, I decided to just mention it here. In the first episode, Mulder and Scully meet a right-wing conspiracy theorist named Tad O'Malley (Joel McHale), who has a popular online show. Because of him, they go back to the FBI and Skinner reopens the long-closed X-Files division. (For years, Mulder had been in hiding, and I have no idea how he supported himself, because I'm pretty sure he didn't have a job. Meanwhile, Scully had been working at a hospital since leaving the FBI. And now I kind of get the impression she kept that job when she rejoined the FBI, though I'm not sure how she finds time to work both jobs.) Anyway, just like the old days, there are mythology eps and monster of the week eps, as well as some eps that were more humorous. I think by far the best episode of the season was the third one, "Mulder and Scully Meet the Were-Monster." I thought it worked very well on a number of levels, though mostly it was a humor episode. (But it's also a very polarizing episode; some people, like me, loved it, while others hated it.) Episode five, "Babylon," introduced a pair of young FBI agents, Miller (Robbie Amell) and Einstein (Lauren Ambrose). Miller is a true believer, like Mulder, while Einstein is a skeptic and a doctor (with red hair, even), like Scully. Though Einstein is a lot less willing to believe any of the weird fringe stuff than Scully ever was. Anyway, they also both appear in the season finale, which finally gets back to the mythology stuff that was set up in the season premiere. We also see Tad O'Malley for the first time since the premiere, as well as special appearances from a couple of familiar characters from the original series. I don't want to spoil anything about the conspiracy revelations from season 10, but I will say I found parts of it interesting and parts of it... not so much. (One thing I did like was the line in the finale about the current plan having been put in motion in 2012, because that was a date that had been mentioned in the original series. And since that year came and went in the real world, after the show ended, without apparent incident, it was nice to know it hadn't been forgotten. Then again, how could it be? Everyone in the real world was aware of doomsday predictions for December 21, 2012, whether they knew anything about The X-Files or not.) Anyway... the episode ends on a cliffhanger, which was incredibly annoying, since we don't know yet whether there will be another season or not. (Either way, it makes it impossible for me to consider the season a "limited series.") And I guess I should say that while the season was perhaps a bit disappointing, I generally enjoyed it, and I'm happy to have seen it. Even the parts I kind of didn't like, I didn't hate the way some people did. (And man, that Were-Monster episode... IMO, that made the whole thing worthwhile.)