London's Slough House is where the washed-up MI5 spies go to while away what's left of their failed careers. The "slow horses," as they’re called, have all disgraced themselves in some way to get relegated here. Maybe they messed up an op badly and can't be trusted anymore. Maybe they got in the way of an ambitious colleague and had the rug yanked out from under them. Maybe they just got too dependent on the bottle—not unusual in this line of work. One thing they all have in common, though, is they all want to be back in the action. And most of them would do anything to get there─even if it means having to collaborate with one another.
Now the slow horses have a chance at redemption. An old Cold War-era spy is found dead on a bus outside Oxford, far from his usual haunts. The despicable, irascible Jackson Lamb is convinced Dickie Bow was murdered. As the agents dig into their fallen comrade's circumstances, they uncover a shadowy tangle of ancient Cold War secrets that seem to lead back to a man named Alexander Popov, who is either a Soviet bogeyman or the most dangerous man in the world. How many more people will have to die to keep those secrets buried?