In this book George Smiley (the retired temporary head of MI6) is asked to investigate the death of General Vladimir (a former spy). Vladimir was a former Russian officer who spied for the British year ago, and lived to retire. The problem was that the General was trying to make contact with MI6, after he is contacted by a Russian emigree in France. The question is, why's he calling? Smiley tries to find out.
As the investigation continues, the death of a "stringer spy" (Otto Leipzig) sees Smiley's concerns confirmed, and he and Karla (the head of the Russian "service") do battle to see if Smiley can come out on top in the third stage of their personal duel.
It's not a bad book, but it feels... old. I can see it's well written, but I think it drags a little (and not just because it's set in a time that doesn't exist any more). The book was written in 1978/79, and in the intervening years I think people have got more used to pace in their books.