Slough House is a dumping ground for members of the intelligence service who've screwed up, left a service file on a train, blown a surveillance or become drunkenly unreliable. They're the service's poor relations known as "the slow horses".
The most bitter among them is River Cartwright whose days are spent transcribing mobile-phone conversations. But when a young man is abducted and his kidnappers threaten to behead him live on the Internet, River sees an opportunity to redeem himself. But is the victim who he first appears to be? And what's the kidnappers connection with a disgraced journalist? As the clock ticks on the execution, River finds that everyone involved has their own agenda.
