The Swamp House is a dumping ground for members of the secret services who have made a mistake that has caused an operation to fail, whether it is forgetting a document on a train, getting lost on a surveillance patrol or becoming unreliable due to alcohol. Their colleagues call them the slow horses. They are the poor relations of British espionage and all share the desire to get out of there at any cost and get back into action.
Of this quirky group of outlaws, the most disenchanted is River Cartwright, who spends his days transcribing intercepted cell phone conversations. But when a young man is kidnapped and the perpetrators threaten to behead him live on the Internet, River sees an opportunity to redeem himself. Is the victim who he appears to be? And what is the relationship between the kidnappers and the disgraced journalist the slow horses are investigating? As the tick-tock that brings us closer to the deadline for the execution sounds, River discovers that each of those involved has hidden interests, and if the slow horses do not wake up, the echo of the crime will spread around the world.