context: In the opening scene, German troops and tanks are shown invading the Kingdom of Yugoslavia while bombers attack the capital Belgrade. When the Germans, Italians, Hungarians, and Bulgarians invade Yugoslavia on 6 April 1941, Serbian army colonel DraÅ¾a MihailoviÄ forms a band of guerrillas known as the Chetniks, who launch a resistance movement against the Axis occupation. MihailoviÄ's forces then engage in an attack on the German and Italian forces, forcing them to employ seven Axis divisions against them.
  The Chetniks capture an Italian supply convoy. MihailoviÄ then radios German headquarters in the nearby coastal town of Kotor in Montenegro and offers to exchange Italian POWs for gasoline. Infuriated, general Von Bauer refuses, but when MihailoviÄ threatens to notify the Italian High Command of his decision, Gestapo colonel Wilhelm Brockner orders Von Bauer to comply.
  Brockner, who has been unable to capture MihailoviÄ, is convinced that the Yugoslav leader's wife Ljubica and their two children, Nada and Mirko, are hiding in Kotor. He plans to use them as hostages to blackmail MihailoviÄ into surrendering. Brockner warns the townspeople that anyone caught aiding the MihailoviÄ family will be executed, and prepares the deportation of 2,000 men from Kotor to Nazi Germany.
  Brockner's secretary Natalia, however, is a spy for the Chetniks and is in love with Alexa, one of MihailoviÄ's aides. Forewarned by Natalia's information, the Chetniks attack the train transporting the two thousand prisoners and free them. In retaliation, Brockner decrees that no food will be distributed to the citizens of Kotor until Lubitca and her children are turned over to the Germans. Lubitca tries to surrender to Brockner but is stopped by Natalia, after which MihailoviÄ asks to meet with Von Bauer and Brockner.
  After MihailoviÄ arrives at German headquarters, however, Von Bauer declares that, since the official Yugoslav government had capitulated, international law does not prevent him from killing MihailoviÄ, even though they are meeting under a flag of truce. MihailoviÄ then reveals to the general that the Chetniks are holding his wife and daughter as hostages, as well as Brockner's mistress, and that they will be executed unless the citizens of Kotor are given food. The general angrily releases MihailoviÄ and provides rations for Kotor.
  MihailoviÄ's son Mirko, demonstrating his patriotism, betrays his true identity to his German schoolteacher. After taking Mirko into custody, Von Bauer and Brockner escort Ljubica to MihailoviÄ's mountain stronghold and then inform him that every man, woman, and child in Kotor would be executed unless the Chetniks surrender within 18 hours.
  MihailoviÄ informs Ljubica that he cannot surrender. She then returns to Kotor to comfort their children. MihailoviÄ immediately organizes a plan of attack and sends some of his men to the mountain pass to Kotor, where they trick the Germans into thinking that they are surrendering, while the rest of the Chetniks attack the town from the mountains on the other side.
  Even though Aleksa, who was assigned to infiltrate the German artillery, is taken prisoner by the Germans, MihailoviÄ's plan succeeds. After an intense battle, the Chetniks gain control of Kotor and free all of the hostages, including MihailoviÄ's family.
  In the final scene, MihailoviÄ broadcasts a radio message to his fellow Yugoslavs that the guerrillas will continue fighting until they have regained complete freedom for their people and driven out the invading Axis troops.
question: How long do the Chetniks have to surrender?
answer: 18 hours