Lancer Spy

In World War I, British naval Lt. Michael Bruce (George Sanders) bears a striking resemblance to a renowned German officer, Baron Kurt von Rohback (also Sanders) who's been imprisoned by the Allies. Bruce's superiors hatch a plan to send him, in disguise, to Germany in order to impersonate von Rohback and gather secrets from his contacts. But the head of the German secret police, Maj. Gruning (Peter Lorre), suspects that the returned Baron is an impostor and so devises a plan to expose him.