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, 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.