It is the end of the Second World War and Lotte and her father Dieter, a recently captured German scientist, arrive in England. Captain Callum Ferguson, an intelligence officer, is assigned the task of courting Dieter into working with the British RAF to develop the jet engine. With the emerging Cold War, it is crucial for British national security that cutting-edge technology is made available to the armed forces as quickly as possible. Meanwhile, Kathy Griffiths from the War Crimes Unit is looking to bring war criminals who escaped prosecution to justice. As Lotte navigates the strange corridors of the bomb-damaged London hotel where they are staying under the British Government's watchful eye, Callum must win over the trust of her father.

Everyone puts aside their differences in the search for Lotte, but Callum is forced to make a difficult choice between his brother and the orders from his superiors.

Believing he is free to both protect and prosecute the Germans as he sees fit, Callum eventually brings Kathy some information about scientists who have been taken to safe houses and given new identities. Kathy and Callum work together to prevent someone important from leaving the country.

With encouragement from her husband Alex, Rachel, an enchanting Anglophile American, gets closer to Callum.