The film is based on the Yugoslav resistance movement under the command of General Draza Mihailovich. But politics overtook the situation because Mihailovich and the Royalists were about to be abandoned by the British government – as parts of the Chetnik movement co-operated with the Nazis – in favour of the Communist and Stalinist leader Josip Broz Tito. Speaking in Parliament on 22 February 1944, the Prime Minister, Winston Churchill, said: "General Mihailovic, I much regret to say, drifted gradually into a position where his commanders made accommodations with Italian and German troops…" The screenplay, by John Dighton and Monja Danischewsky, was accordingly amended, and the film was re-edited. It ended up as a black and white war film, 80 minutes in length.

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.

• Director: Sergei Nolbandov
• Producer: Michael Balcon
• Associate Producer: S.C. Balcon
• Script: John Dighton, Monja Danischewsky, Sergei Nolbandov (uncredited), and Milosh Sekulich (uncredited). Based on a story by George Slocombe, Milosh Sekulich (uncredited), and Sergei Nolbandov (uncredited)
• Cinematography: Wilkie Cooper
• Art Direction: Duncan Sutherland
• Editing: Eileen Boland
• Supervising Editor: Sidney Cole
• Special Effects: Roy Kellino
• Technical Advisors: Milosh Sekulich, W.E. Hart
• Music: Frederic Austin

• Barr, Charles. Ealing Studios: A Movie Book. Berkeley, CA: University of California Press, 1999.
• Barr, Charles. (1974). "Projecting Britain and the British Character: Ealing Studios, Part II." Screen, 15(2), pages 129-163.
• Dick, Bernard F. The Star-Spangled Screen: The American World War II Film. Lexington, KY: University Press of Kentucky, 2006. p. 164.
• Undercover on the citwf database: http://www.citwf.com/film365195.htm

• Undercover at the British Film Institute^{[better source needed]}
• Undercover at IMDb