25 years ago, young Suzy Carter suddenly died after falling out of the window of a hotel room, apparently committing suicide.
She was there under police protection, as she was about to give vital evidence in a corruption case that would have incriminated a successful & well-respected businessman.
But, in present day 1978, the head of MI6's last words as he lies on his deathbed are 'I killed Suzy Carter'.
Cowley is approached, as it is felt his squad are best-placed to find the truth of what happened... and are the least corruptible.
But, it seems there's someone out there who doesn't want the the truth to be found -and soon everyone who was involved in Suzy's case back in 1953 are being picked off one by one by a professional and elusive hitman.
Not only that, but all records of the historic case seem to have mysteriously gone missing...
This is a good, solid story from the first season, and offers a slightly different approach - what with the pre-credits sequence being set so far back in the past.
Despite the sombre plot, there's still plenty of room for humor - Bodie & Doyle's scenes with new CI5 recruit Tony are hilarious, as they chide him for referring to Cowley as 'The Cow' and then play a childish prank on him in the locker room (at Cowley's expense) - only to be caught out in the act by The Cow himself like naughty schoolboys.
Trying to make actors looks 25 years younger in the opening sequence perhaps isn't entirely convincing - maybe it would have been wiser to have made this event 10 years ago or so (although the year of 1953 is what gives the episode its title, & is referred to throughout).
There's a good scene early on where the boys confront the corrupt Turvey in his mansion, and get short thrift (Bodie in particular being on the receiving end of Turvey's anger) - but you get the feeling this spurs him on even more to find the evidence they need to nail him.
This is an enjoyable tale of betrayal and retribution, and of the need to bring those to book who thought they had gotten away with their crimes of the past.