You've got to love the British for making one of their best TV shows about domestic security (MI-5) rather than international intrigue (MI-6). You've got to further love the fact that the British title for the show is "Spooks," which is evidently what they call their spy types in the UK. Cheeky buggers.
MI-5 started in 2002, and ran for ten seasons, through 2011. When Mr. CR and I first started watching it, it was fairly early on in my British TV-watching career, and we were really surprised and pleased at the complexity (both in plot and moral tone) of it, as well as with the slickness of the production.* One of our favorite things about it was its periodic use of screen-in-screen action to show how storylines were developing at different times or in different places. It really felt like they were packing a lot of story in a one-hour program.
Once again the cast proved to be almost uniformly excellent, and even though the writers/producers were never afraid to kill off main characters, often very surprisingly**, you never even had the chance to miss your favorite characters because they were often replaced by equally interesting ones. Matthew Macfadyen as Tom Quinn (excellent) became Rupert Penry-Jones as Adam Carter (excellent) became Richard Armitage as Lucas North (excellent). And don't even get me started on the mainstay actor of all ten seasons: Peter Firth, who played Harry Pearce, a Section Head (the big honcho, basically) within MI-5. So, so good.
The series did start to lose a bit of steam in the later seasons, but it was still very watchable, and god love the BBC, their series usually only run about six to twelve episodes or so, so even a ten-season show doesn't take as long to watch in its entirety as you might think.
Now, if you watch this trailer for the show, I think you'll think it looks ridiculously violent. And you'd be right; there's violence in the show. But it never really felt like it glorified violence: a lot of times the only choices open to the characters were bad ones. Of course a show about a national spy service is going to be nationalistic ('for the greater good,' etc.) but it certainly didn't make the lifestyle look glamorous. I liked that.
*And we stayed such nerdy fans, as a matter of fact, that we tried to stop by the real "Thames House," where MI-5 is based, when we traveled to London.
**One of the rather graphically displayed deaths in the first season actually led viewers to contact the BBC in protest.