The first three seasons were brilliant. Dazzling, laser sharp dialogue and tightly-woven narrative, sympathetic, close knit characters, fine acting by some of Britain's best, mordant humor that blew by so quickly it was hard to catch, and suspense that had us addicted and desperate for the next episode. We watched all three seasons in a matter of days last summer. Since then, I've referred to MI-5 as "24" for brains. The show wasn't just about chasing terrorists. It had drama and depth and in the course of events raised questions about the age we live in and the difficult choices we make.
Now the main three characters are gone, with the exception of the excellent Harry, played by Peter Firth (I saw him in "Equus" on stage many years ago). Harry has a strong moral center. He lives in the past when having said moral center mattered to an idealistic lot. He provides an avuncular but hard-boiled anchor to the work of MI-5.
So, now that the first three have been dealt with, each in their own unpredictable way, we have new characters. They are appealing to be sure, and don't lazily fit into the old formula, but none measure up individually or as a unit to the threesome of the past. We care less about them. They are more thinly drawn.
The writing seems to have suffered a bit. I'd call it smart, but not brilliant except in an occasional flash. The humor is harder to place, and there are traces of a plot line getting desperate.
Still, it is riveting. One of the interesting things about a counter terrorism program made in and set in Britain is the different perspectives you get on things, like torture. MI-5 doesn't patronize, or glamorize, or fetishize it, but neither does it back away from the reality that it is an attractive tool even outside today's USA.
Speaking of which, there are lots of references to "you'll talk because otherwise you know what we'll do: turn you over to the Americans."