Yes, Prime Minister: Series 1 (1986) | One of Us (Episode 8)

Episode 8: The former head of MI5 is revealed to be a spy (despite Sir Humphrey clearing him), while a dog strays onto Salisbury Plain. Featuring Michael Aldridge.

Series 2: https://rumble.com/v6wju0k-yes-prime-minister-series-2-198788-man-overboard-episode-1.html