Slough House - the crumbling office building to which failed spies, the 'slow horses', are banished - has been wiped from secret service records. Reeling from recent losses in their ranks, the slow horses are worried they've been pushed further into the cold, and fatal accidents keep happening. With a new populist movement taking a grip on London's streets, the aftermath of a blunder by the Russian secret service that left a British citizen dead, and the old order ensuring that everything's for sale to the highest bidder, the world's an uncomfortable place for those deemed surplus to requirements.
The wise move would be to find a safe place and wait for the troubles to pass. But the slow horses aren't famed for making wise decisions.