If you're a fan of British espionage and spy thrillers then MI-5 is for you. I came across it over the holiday and it's just a dynamite series. It follows Britain's MI-5, their Secret Service which offers a rich drop of plots filled with twists and turns, global intrigue and excellent character development. It began its run in 2003 and it's fascinating to watch how UK writers broach the rest of the world's security problems that are so closely linked to all of Europe and the Middle East.
It really takes off after the first season and surprisingly never loses its edge even as actors come and go. They handle their protagonists quite differently than American TV. They don't always treat Americans favorably and there are episodes before Bush invaded Iraq that show how much the world changed after his preemptive doctrine speech. They are on their 8th season and it's just as interesting as its ever been.