In Northern Rhodesia Lady Felicia lies in bed with Benedict Lewis. His return to England imminent, Lady Felicia finally decides to join Benedict and tell Monty that she's leaving him. Along with Sid they surprise a delighted Father Brown, Mrs McCarthy and Bunty at the Presbytery. Later Lady Felicia is intercepted at the train station by her old adversary Daniel Whittaker, who explains that Benedict is a soviet spy and important blueprints have been stolen from the Ministry of Defence. He anticipates that an exchange of the film will take place at Benedict's fundraiser and gives Felicia a fake film to replace with the real one. Lady Felicia is left not knowing who to believe. At the fundraiser, Alfred Lane presents Benedict with a sculpture and Lady Felicia suspects the film is hidden inside. As she attempts to swap the films over Benedict appears and points a gun at her - has Lady Felicia really fallen for the enemy?
