"Close to the Enemy" follows intelligence officer Captain Callum Ferguson (Sturgess), whose last task for the Army is to ensure that a captured German scientist, Dieter (Diehl), starts working for the British RAF on urgently developing the jet engine. With the background of the emerging Cold War, it is clear to all that it's crucial for British national security that cutting-edge technology is made available to the armed forces as quickly as possible. Callum uses unorthodox methods in his attempt to convince Dieter to work with the British and eventually a friendship develops between the two men, but soon tensions arise as all is not as it seems.
Over the course of the series, Callum encounters a number of other characters whose stories all intertwine. These characters include Victor (Highmore), Callum's younger brother, struggling to deal with psychological trauma caused by his experience in the fighting; Harold (Molina), a Foreign Office official who reveals some startling truths about the outbreak of the war; Rachel (Riley), an enchanting Anglophile American engaged to his best friend; and Kathy (Fox), a tough young woman working for the War Crimes Unit, fighting to bring war criminals who escaped prosecution to justice. All these characters are trying to rebuild and move their lives forward in the aftermath of the war, a war that scarred them all so deeply.