It is 1937, and Colonel Jean-Francois Mercier, a highly decorated soldier from the First World War, is the military attaché stationed in Warsaw, Poland while the shadow of Hitler’s Nazi regime looms over mainland Europe. Mercier is grossly suspicious of the German military’s intentions, but he must juggle his formal duties at stifling diplomatic functions with the often death-defying realities of espionage. With intelligence from his German informant Edvard Uhl, Mercier sets out on a stealth mission with his trusted accomplice Marek, where they discover that the Nazis are ensuring any future invasion of Poland can take place effectively and with little resistance. Mercier almost gets caught and killed during the mission, narrowly escaping, and thereby facing the wrath of French bureaucrat Jourdain, who is anxious to avoid the further heightening of diplomatic tension. Amid the threat of a second world war, Mercier is also vying for the affections of Parisian lawyer Anna Skarbek, currently in an empty-shell relationship with Russian Émigré and political journalist Maxim. As Mercier and Anna’s relationship develops, it is clear that she must choose between both men. But will Mercier’s feelings for Anna jeopardise his mission and can she be trusted?

Mercier’s mission becomes all too dangerous as his cover is compromised while trying to unearth German military plans, and a man believed dead reappears, sparking significant repercussions for both Mercier and Anna. Will Mercier discover the truth in time and what consequences will this have on the rest of Europe? Led by intelligence from Dr Lapp, Mercier travels to Czechoslovakia to find and question the illusive ‘Seagull’, a German activist who is on the run from the Nazis. Finding Seagull reluctant to become involved with the French, Mercier soon takes matters with into his own hands and persuades him to travel to Berlin under false identities. There, with Seagull’s help, Mercier hopes to employ a man who could prove to be a powerful ally against the Nazi regime. As Mercier’s superiors become increasingly frustrated with his lack of protocol and risk-taking, they begin to question the intelligence he has gathered, when finally, Mercier finds himself in opposition to his own government. Frustrated with work and longing for the beautiful Anna, he returns to Paris and the support of his younger sister Gabrielle (Tuppence Middleton), eventually seeking solace at the old family estate and his father’s well-stocked wine cellar. Now the threat of war becomes all too real as tensions rise across Europe and Mercier has to make difficult decisions.