A British intelligence officer must persuade a German scientist to work with the RAF.

Callum uses unorthodox methods in his attempts to convince Dieter to work with the British, while juggling the demands of Kathy, an officer in the War Crimes Unit, and his troubled younger brother, Victor. Kathy's team are concerned that the British government is harbouring war criminals under the name of science, while Victor struggles to deal with the psychological trauma caused by his experience of fighting the war. Kathy and Victor seek Callum's help, but can they trust that he will put their aims before his own? The power play between Dieter and Callum eventually puts Lotte at risk.

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.

Kathy and Callum successfully prevent a war criminal from leaving the country, but is he too important to prosecute? Callum is instructed to get rid of all information pertaining to individuals of interest, while also having to court the imperious Frau Bellinghausen who holds the key to an infamous perfume. Callum juggles these new demands as his relationship with Rachel intensifies and becomes more compromising. Victor and Kathy make the most of Callum's distractions to find whatever his department has been hiding. Callum begins to realise that he is not the free agent he has always believed himself to be. Callum's dealings inside the hotel bring him closer to Harold, an ex-Foreign Office official and hotel regular, who reveals some startling truths about the outbreak of the war. Meanwhile, the friendship between Dieter and Callum is threatened as it is believed that Dieter may have stood by as innocents in labour camps perished under the rule of the Nazi party.

While Dieter is seemingly helping the British to achieve their jet engine goals, Victor comes to Callum with proof that Dieter isn't innocent of war crimes. After thwarting Victor and Kathy's attempts to recover information by stealth, Callum regains their favour by helping Kathy get access to a useful German prisoner.

Victor, still struggling to cope with the effects of the war, tries to expose the activities of Callum's team. Those around him choose not to pay attention and force him to seek medical help. He escapes from the hospital psychiatric ward at an opportune moment, while Callum and Rachel take advantage of Alex's upcoming business trip. When Callum's superiors try to stop Victor, his life is threatened. Dieter finally tells Callum the truth about his past in Germany. The tables have turned, and it is now Dieter who tries to win over Callum. Harold similarly feels despair for failing to live up to Callum's expectations of him. Can these two men convince Callum of their integrity?

Preparations for Dieter's wedding are underway. Employing the same tactics as Callum, Kathy finally confronts Bergit about her activities, forcing her to realise that her options are limited. Meanwhile, Harold makes a decisive move as he struggles to cope with feelings of inadequacy. Concerned that he is caught in a marital game, Callum forces Rachel to admit her true feelings for him. As ever, Victor remains haunted by the effects of the war and the people who will get away with crimes they committed in the name of war. He ultimately learns that there are some people who are too important to prosecute no matter what they have done.