Slough House is a dumping ground for British intelligence agents who've screwed up cases in any number of ways - by leaving a secret file on a train or blowing a surveillance. River Cartwright, one such "slow horse", is bitter about his failure and about his tedious assignment transcribing cell phone conversations.

When a young man is abducted and his kidnappers threaten to broadcast his beheading live on the Internet, River sees an opportunity to redeem himself.

Is the victim whom 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 has his own agenda.