This is the first time I have gotten involved in trying to watch 10 seasons of anything. Thank goodness for the library. I watched Season One from the shelf of my favorite library. It ended with the cliffhanger of all cliffhangers and I wanted more. BUT my library didn't have the rest of this series in 10 parts. Thankfully, they have been able to get subsequent seasons thru inter-library loan. I just finished Season 5 and Season 6 has been ordered. It's the most exciting thing I have EVER watched. I love British Spies, you know!
The acting is superb and the head of this counter intelligence agency is Peter Firth and he's wonderful. I enjoyed seeing him last night in the finale of Victoria on PBS. He played Duke Cumberland.
The actors come and go and are replaced by equally superb characters.
Have you seen this series? It was on PBS, I believe. It started in 2002. Anyway, it's on Hulu if you subscribe and it might be on Acorn. Thankfully, my library has been able to find them all for me, so far. Each season is very expensive to buy, selling for as much as $80 per season.
It is/was called
SPOOKS in the UK.
Look for it. You will NOT be disappointed.