In 1942, in German-occupied Russia, Russian engineers work under cover of darkness to build a bridge beneath a river facing German lines, over which Russian tanks plan to launch a counter-attack. In preparation for the attack, a group of Baltic paratroopers are assigned to penetrate enemy lines and ascertain the position of enemy reserves. At headquarters, the troopers meet Kostyuk, the commander of the operation, and marvel as he shrewdly interrogates a German prisoner. After Kostyuk introduces them to their guide, Lisa Elenko, the soldiers parachute into a factory near Lubovonov, which the Germans are using as divisional headquarters. The Russians capture their objective, but in the raid, the factory is severly bombed and collapses. In its subterranean cellar, only Lisa, Alexei Kulkov, one of the paratroopers, and seven German soldiers survive. After taking the Germans prisoner, Alexei finds a German officer's revolver among their possessions and determines to unmask the officer hiding among the enlisted men. Above ground, in the ruins, Alexei's dog claws at the rubble while the sole surviving Russian taps a message to those below. When Alexei responds in code and instructs the Russian to report that a German officer has been captured, the Germans scoff at Alexei's claim until he silences them by showing them the revolver. As Alexei interrogates the Germans, the dog and the Russian steal through the night across enemy lines. When the Russian is killed by German bullets, Alexei's dog continues his trek to headquarters. Meanwhile, in the cellar, Alexei questions Mueller, a professor in civilian life, who begins to rant about an anti-German book that he has written. When Alexei becomes mesmerized by Mueller's diatribe, the Germans attack. Although Lisa and Alexei fend off their attackers, Lisa is stabbed in the assault. In retaliation, Alexei announces that he intends to kill the Germans, one by one, until he finds their commander. After ordering the sergeant behind a wall, Alexei knocks him out and then fires his gun, causing the Germans to believe that he has killed him. As Alexei confronts his next victim, the man defiantly proclaims that he is Major Ernemann. When the sound of tanks rumble above ground, the major taunts Alexei with images of imminent defeat and suggests that if he is so certain of Russian victory, he should have no qualms about exchanging invasion plans. Alexei, determined to elicit the location of the concentration of German forces, offers to trade information with the major. After the major pinpoints the site of the German tank division, Alexei shows him the Russian crossing point in the river. In exchange for the name of the village in which the German troops are to assemble, Alexei tells the major about the underwater bridge. Lisa, shocked by Alexei's revelation, urges him to kill the Germans before they can use the information. Alexei rejects Lisa's advice, claiming that he must first ascertain whether the major has told the truth. As the bridge nears completion, Alexei's dog reaches headquarters and then leads the troops across the bridge to his trapped master. Meanwhile, in the cellar, Alexei pretends to fall asleep and one of the Germans awakens him and offers to be his ally. Alexei then hands the German a rifle and drifts off to sleep. Later he is awakened by the sound of digging. When the major charges toward their rescuers, shouting the Russian plan of attack, Alexei shoots him. Thinking that the German army is breaking into the cellar, Alexei is about to pull the pin in his grenade when he sees his dog and the Russian army burst through the rubble. After Lisa is carried out on a stretcher, Alexei turns over his prisoners and reports the details of the German invasion plan. Taking the rifle from the German's hands, Alexei reveals that it holds no bullets and then collapses from fatigue.
Output: Question: What must Kulkov do by himself?, Answer: unmask the officer hiding among the prisoners