Slow horse River Cartwright is waiting to be passed fit for work. With time to kill, and with his grandfather - a legendary former spy - long dead, River investigates the secrets of the old man's library, and a mysteriously missing book. Regent's Park's First Desk, Diana Taverner, doesn't appreciate threats.

So when those involved in a covert operation during the height of the Troubles threaten to expose the ugly side of state security, Taverner turns blackmail into opportunity. Over at Slough House, the repository for failed spies, Catherine Standish just wants everyone to play nice. But as far as Jackson Lamb is concerned, the slow horses should all be at their desks.

Because when Taverner starts plotting mischief people get hurt, and Lamb has no plans to send in the clowns. On the other hand, if the clowns ignore his instructions and fool around, any harm that befalls them is hardly his fault. But they're his clowns.

And if they don't all come home, there'll be a reckoning.