It is late summer 1934 when retired MI6 agent John Repton's body is found near Wyndham Hall in the Cotswolds. Repton was killed while investigating the Wyndham family's ties to fascist sympathisers, and Elena Standish is assigned to discover if one of them resorted to murder. Meanwhile, Elena's sister Margot is courting Lady Wyndham's brother and, unaware that Elena is a spy, Margot invites her to attend a house party at Wyndham Hall along with her colleague James Allenby, who masquerades as her suitor. As the atmosphere becomes increasingly tense, Elena and Allenby begin to expose the allegiances of the people in the house. But Elena is torn, for she knows that revealing the truth will protect the nation's security but could potentially destroy her sister's happiness.