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

Callum uses unorthodox methods in an attempt to convince Dieter to work with the British, while also juggling the demands of War Crimes officer Kathy — who believes the British government is harboring war criminals — and his troubled younger brother, Victor.

Everyone puts aside their differences in the search for Lotte and Callum is forced to take a choice between his brother and 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.

The dramatist Stephen Poliakoff has long been obsessed with the secret history of Britain in the 20th century. His latest work, Close to the Enemy, looks at the clandestine work of the secret service after the end of the Second World War.

Historian and broadcaster David Reynolds talks to Poliakoff about the inspiration behind Close to the Enemy, as well as the always tricky relationship between history and fiction.

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 goes some way to regaining their favour by helping Kathy get access to a useful German prisoner, Birgit Mentz. He advises Kathy to try employing his own tactics to get Birgit to open up. Callum thinks he can get the proof that Harold requires to expose the British government's wartime secrets, and also keep Victor and Rachel close to him at the hotel. But can he have it all?

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?

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?