British spy Leonard Flood is asked to investigate the poisoning in London of Willa Karlsson, a retired British secret service vetting officer suspected of being a Russian agent. British intelligence is terrified by the possibility that Moscow poisoned her upon her retirement since she was no longer useful to them. 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.