River Cartwright is ousted from MI5 and finds himself in a place worse than purgatory: Slough House, dumping ground for failed spies.

Slough House is the unlikely venue for a secret romance, but events take a dark turn when a dangerous encounter becomes deadly.

Taverner sends the Dogs to hunt down Lamb and the Slow Horses. River holds the secret to saving the gang.

The Slow Horses must disappear to avoid capture by MI5. Lamb and River head to The Park to outwit Taverner.

Taverner and the Slow Horses attempt different approaches to locate the kidnappers and Hassan. Ho makes a shocking discovery about Sid.