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 paranoid concerns of David Cartwright, a Cold War-era operative and one-time head of MI5 who is sliding into dementia, and questions his grandson, River, must figure out answers to now that the spy who raised him has started to forget to wear pants. But River, himself an agent at Slough House, MI5's outpost for disgraced spies, has other things to worry about. A bomb has detonated in the middle of a busy shopping center and killed forty innocent civilians. The "slow horses" of Slough House must figure out who is behind this act of terror before the situation escalates.