This is another proper spy book. And it starts by stating the obvious. There's a traitor in the MI6, and now we follow a bunch of characters around, trying to figure out who among it is. You know the players, you know what they do, and you suspect every action. As typical of the genre of that type, there's a lot of geo-hopping, something that James Bond established in the previous decades, so you need at least a dozen exotic yet weird locations to support the plot. This was before you could go on Google Maps and see what gives for yourself.
We will also get to see the charming Tweed character in several other books. All in all, laid-back rainy Sunday kind of thriller book, the modern day equivallent of Poirot's mysteries. Wossname. You get the idea.
There was a man named Tweed,
Double agents he set to mislead,
About face,
National disgrace,
For hunting spies was his creed.
Cheers,
Comrade Igor McTrustyface