Lamb and his crew get involved investigating why a retired agent ends up dead on a bus transporting rail passengers from a train station with a problem. Russian agents, a 1980’s rumor of a sleeper cell of cicadas. Min Harper and Louisa Guy are co-opted to assist with Spider Webb’s plan to nurture a man he believes to be a Russian agent. Lamb sends River Cartwright to a small a town in Gloucestershire to determine why the dead agent was headed that way. 
Herron makes some abrupt switches in character which sometimes makes the narration seem to skip a beat - but I was still able to follow the story line. In the end -

Cartwright once again calls what is a false alarm of a terrorist attack in London, allowing the fake Russian agent to steal diamonds from businesses in the building that is being evacuated. Webb is shot but lives - Min Harper is killed earlier in the book - leaving Louisa seeking answers - and revenge. River learns that Old Tom is the true Russian agent - who has recruited people to live in the small town - waiting for their orders. Except - Tom is the only one who complies