UNDERCOVER (1943)

Undercover is a 1943 British war film produced by Ealing Studios, originally titled Chetnik. It was filmed in Wales and released on 27 July 1943. Its subject is a guerrilla movement in German-occupied Yugoslavia, loosely based on Draza Mihailovich's Chetnik resistance movement.
The film focuses on the fictional Petrovitch family in Belgrade, Serbia. One brother, Milosh, a Yugoslav military captain forms an anti-Nazi guerilla movement in the mountains of Serbia. His brother, Dr. Stephan Petrovitch, poses as a Nazi collaborator to obtain information for the guerrillas while working directly under General von Staengel, commander of the German occupation force.
Using information obtained by Stephan, Milosh and his guerrillas are able to ambush a German train and free Yugoslav PoWs, while wounding General Staengel in the process. Stephan operates on the wounded General, saving his life, and gaining the General's trust. Milosh's wife, Anna Petrovitch, a schoolteacher, is taken prisoner and interrogated, but she escapes, with the help of some of her students, and joins Milosh in the mountains. In retaliation, German troops under Colonel von Brock execute six schoolchildren.
Later, Stephan uses his credentials as a Nazi sympathizer to plant explosives on a German train, timing them to go off in a mountain tunnel. The film's climax is a pitched battle between the Germans and guerrillas. Afterwards, the Serbians retreat into the mountains to continue their campaign of terror and resistance against Axis occupation.