Autumn 1943. Gromov's reconnaissance group is attacked by an enemy shepherd dog during a sortie. In defense, the soldiers open fire for protection, but this action gives away their location and they come under mortar fire. Afterwards, Gromov takes the dog with him and asks surgeon Vasilev to operate on it and save the animal. When the dog recovers, Gromov tries to establish contact with it, but he fails. Only Corporal Miroshnikov helps the hero find a common language with the dog; even Gromov's girlfriend, nurse Mariya, disapproves of Viktor's idea. But Gromov is determined to re-educate the dog and make a real Soviet soldier out of it. He gives the shepherd the nickname Reks. Soon, thanks to his kindness and resourcefulness, he manages to make the dog an assistant, ready to go with him on reconnaissance and carry out difficult tasks. But one day Gromov meets Reks's former owner, who tries to make the dog remember his old habits and commands.

