MI5's First Desk, Diana Taverner, doesn't appreciate threats. So when those involved in a covert operation during the height of the Troubles in Northern Ireland threaten to expose the ugly side of state security, Taverner goes on the offensive. And when that happens, people get hurt.

Over at Slough House, the repository for failed spies, the slow horses are hard at work: belittling each other, pushing paper, and marking time. Their boss, Jackson Lamb, wants them at their desks, but the slow horses are always looking for a way back on to Spook Street. Meanwhile, another slow horse, River Cartwright, is waiting to be passed fit for work.

At a loose end, 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. What stories will it hold? And who wants those stories to stay silent? After all, spies lie, they betray, it's what they do.