At the end of WWII, British intelligence officer Callum Ferguson is assigned to persuade captured German scientist Dieter Koehler to work with the British RAF to develop an urgently needed jet engine.

Callum employs unorthodox methods to convince Dieter to work with the British, while also juggling the demands of War Crimes officer Kathy - who believes the British government is harbouring war criminals - and his troubled younger brother, Victor.

Everyone must put aside their differences to search for Lotte, and Cullum is forced to make a choice between his brother and following orders from his superiors. Callum's best friend, Alex, encourages his American wife, Rachel, to seek Callum's help in becoming a patron of the arts.

Callum struggles to keep up with his work responsibilities and the demands of the hotel's newest guest, Frau Bellinghausen, while his relationship with Rachel becomes more compromising. Callum gets to know Harold, an ex-Foreign Office bureaucrat, who reveals some startling truths about the outbreak of the war.

While Dieter is helping the British achieve their jet engine goals, Victor provides information to Callum showing that Dieter may not be as innocent as he seems. Then, Callum works to regain favour with Kathy by helping her get access to a useful German prisoner.

Callum and Rachel take advantage of Alex's upcoming business trip, while Victor tries to expose the criminal activities of Callum's team. Dieter finally opens up about his past in Germany.