From NetFlix:

This award-winning series dramatizes the professional exploits and personal lives of the “spooks” of MI-5, the British equivalent of the FBI. No-nonsense head officer Harry Pearce (Peter Firth) guides a band of dedicated spies who risk their lives every day. The team includes expert and junior members who must balance real life with the job’s requirement for complete secrecy, no matter the cost. Matthew Macfadyen and Richard Armitage co-star.

Beginning in 2002 and continuing at least to 2010, this British TV-series is “utterly smashing”. Intense, sometimes hard to watch, but incredibly exciting in just about every episode. What makes some episodes difficult to watch is that the members of MI-5 are forced at times to make hard decisions that involve “collateral damage” (meaning some poor soul dies), so much so that those members are often damaged themselves and actually leave MI-5 (i.e. leave the show). In fact, be prepared for many, sometimes disappointing cast changes. For example Rupert William Penry-Jones (the character Adam Carter) started the show and then disappeared for awhile to be replaced by Matthew Macfadyen (the character Tom Quinn) who then leaves the show and is replaced by Rupert William Penry-Jones.

There are too many wonderful characters to even mention them. You can find them all listed in IMDB.

In addition to spook plots, there are also many romances. But any romance for a member of MI-5 is next to impossible because even their significant others cannot know what these spooks do for a living.