The Bond-esque River Cartwight and his group of defunct MI5 spies, headed by the irascible Jackson Lamb, will do anything to get back into the game. When a member of London's Slough House - MI5's stable for disgraced spies, so-called "slow horses" - is kidnapped by a former soldier bent on revenge, the agents must risk treason and breach Regent's Park to steal intel in exchange for their comrade's safety. But the kidnapping is only the tip of the iceberg as they are caught in a conspiracy that threatens the future not only of Slough House but of MI5 itself.