They've done ridiculously stupid things or pissed off ridiculously powerful people (or both). Some of them are out and out incompetent and others made a single but unforgivable mistake. They are then sent to an out of the way, dingy, office called (unofficially) Slough House. This leads to the nickname "Slow Horses." There they sit, watching their careers die.
Of course the book is not about a bunch of wayward spies sitting around counting paper clips until retirement. There is SPY STUFF going on and lots of intrigue, backstabbing and hilarity.
Dead Lions is a continuation of the espionage (and pratfalls) of the denizens of Slough House, necessitating that they, once again (sort of) save the day. Old spies from days gone by play a part in this installment.
It is rare thing when I get enthusiastic about a SERIES of books that is more than two or three books but these Slough House books? I have to hold myself back from reading the next one because, while i want to read and read and read? I don't want them to be over. At the moment I am poised before the third book, Real Tigers after finishing Dead Lions.
I will ration them out over time!
Also? The Apple TV show based on these books, starring Gary Oldman and Kirsten Scott Thomas is also pretty good. It diverges enough from the books to keep from being simple rehash as well. It also gives fans of the book something to talk about; "so and so is NOT who I pictured in that roll" or "so and so is exactly who I pictured farting his way through that role!"