The Slough House crew becomes entangled in a plot to discredit the Prime Minister and shut down their operations. But - the people who dreamed up this scheme grossly underestimated Jackson Lamb. My favorite Lambism from this book - when Lady Di is meeting with Jackson to tell him about the plan to disband Slough House he says - “You can’t - I have hostages” -“They are employees, Jackson” “Same difference”
Catherine Standish is kidnapped - and picks River Cartwright as the one co-worker she would trust with her life. This begins a chase fo River to gain access to Regents Park and bluff his way into retrieving some “grey files” in exchange for Catherine. He gets in, but is unable to retrieve the files. Jackson and Diana T collaborate - but in the end Jackson and his crew figure out where the files really are. After firing Shirley and Marcus - they decide to keep investigating. They end up in a basement location with one lone MI5 employee - while River and Louisa Guy are heading to the same place - along with the private Tiger Team that I believe is connected to the Prime Minister. The Slow Horses persevere - and Jackson and Min Ho find Catherine and secure her release. 
At the end -

the Tiger Team sends someone to Slough House to close up a loose end - and kill Jackson Lamb. Herron goes through this big description of the “ghost” coming in to the house - heading upstairs to an office - listening to Jackson Lamb snoring. Just when the ghost thinks this will be too easy - they open the door - and Lamb shoots them.