The series is based on one of John Le Carré's most famous books with the same name, "Tinker, Tailor, Soldier, Spy", which in fact is the first book in a trilogy.
In my opinion this is the best television series ever made in its genre and I don't have the number of how many times I have seen it. In order to enjoy it fully and not loose the thread in the plot you should be able to se all episodes continuously, because it is not always easy to follow John Le Carré in all his twists and turns. Thus my recommendation is that you find a day with troublesome weather so it is cozy to stay inside, lit a fire in the fire place, find something nice to eat and drink and just let yourself be a part of George Smiley hunting down the mole within the "Circus". The actors, the story, filming locations and scenery are all perfect in the way that they produce a fantastic atmosphere where the action takes place. You can almost smell the dust and stalled air inside the "Circus" long corridors and small offices. Some of the Eastern European scenes were actually shot in Scotland.
The series was made for television in 1980 so both the picture (4:3) and sound quality has a certain "seventies" finish. However, in my opinion this just make the series even more authentic.
Buy this series and you will never regret. My last recommendation is that you buy the series based on the third book in the trilogy as well, "Smiley's People". Even though it is a little bit more "modern" it is just as good. The second book in the trilogy, "The Honorable Schoolboy" was for some reason never filmed.
I am a devoted John Le Carré and George Smiley fan, so every time I have my annual trip to London I go to Kings Road in Chelsea in order to visit George Smiley's house in Bywater Street.
Thank you for your attention!