A shakeup at MI5 and a terrorist attack on British soil set in motion clandestine machinery known to few modern spies. David Cartwright isn’t a modern spy, however; he’s legend and a bonafide Cold War hero. He’s also in his dotage and losing his mind to Alzheimer’s. His stories of “stotes” hiding in the bushes, following his every move have been dismissed by friends and family for years. Cartwright may be losing track of reality but he’s certain about one thing: Old spooks don't go quietly and neither do the secrets they keep.

What happens when an old spook loses his mind? Does the Service have a retirement home for those who know too many secrets but don’t remember they’re secret? Or does someone take care of the senile spy for good? These are the questions River Cartwright must ask when his grandfather, a Cold War–era operative, starts to forget to wear pants and begins to suspect everyone in his life has been sent by the Home Office to watch him.

But River has other things to worry about. A bomb has detonated in the middle of a busy shopping center and killed forty innocent civilians. The agents of Slough House have to figure out who is behind this act of terror before the situation escalates.