Leonard Flood, a British spy, is ordered to investigate the poisoning of a retired vetting officer named Willa Karlsson. After a series of recent security breaches (recounted in the first two novels in the series), British intelligence is terrified by the possibility that Willa Karlsson was in fact a Russian agent, and that Moscow poisoned her upon her retirement since she was no longer useful to them. Leonard - intense, prickly, awkward but a brilliant interrogator - throws himself into the investigation, searching Willa's home and questioning her friends and neighbours. He discovers that she was a regular visitor to a remote countryside hotel. Accompanied by a young colleague called Franny, Leonard travels there and discovers to his surprise a long-term Russian agent (known as an illegal) living undercover as the hotel's gardener. When Leonard discovers that he is also a suspect in the investigation and that Willa's story is less a story of betrayal than one of friendship, he must decide whether to hand her to her masters or to help her to escape.
