London's Slough House is where disgraced MI5 operatives are reassigned to spend the rest of their spy careers pushing paper. But when one of these “slow horses” is kidnapped by a former soldier bent on revenge, the agents must breach the defenses of Regent's Park to steal valuable intel in exchange for their comrade's safety. The kidnapping is only the tip of the iceberg, however, as the agents uncover a larger web of intrigue that involves not only a group of private mercenaries but also the highest authorities in the Security Service. After years spent as the lowest on the totem pole, the slow horses suddenly find themselves caught in the midst of a conspiracy that threatens not only the future of Slough House, but of MI5 itself.