Felicia and Sid return from Northern Rhodesia and visit Kembleford while her husband, Monty, is in London. Felicia finds herself blackmailed by a MI5 agent, Daniel Whittaker to exchange a roll of film one of her lovers, a known soviet agent, has. At a party her task goes wrong when a sculptor is shot dead and Whittaker orders Mallory to find Felicia as the prime suspect. Father Brown becomes involved with spies, British and Soviet, not knowing who is friend or foe, traitor or patriot. Lady Felicia has to choose between lover and husband and Moscow get the film they do not expect.