The teenager Misha Polyakov, being with his mother in a Ukrainian village on vacation with his grandmother and about to return home to Petrograd, offers his best friend Genka to go with him. Suddenly, a white gang of Nikitsky bursts into the village and attacks Misha’s house, where Commissioner Polevoy lives. The purpose of Nikitskiy (aka Nikolskiy) is a dagger located at Polevoy. Saving the commissioner, Misha learns from him the history and secret of the weapon, and receives it for storage with a request to solve the secret. Having returned with adventures to Petrograd, Misha Polyakov with friends begins to unravel the riddle of the dagger, in the handle of which is encrypted text.

From childhood, Sanya Grigoryev was able to achieve success in any business. He grew up a courageous and brave man. The dream of finding the remnants of Captain Tatarinov’s expedition led him to the ranks of polar explorers. The life of Captain Grigoryev is full of heroic events: he flew over the Arctic, fought against the Nazis. He was in danger, had to endure temporary defeats, but the hero’s persistent and purposeful character helps him to keep his vow made to himself in childhood: “Fight and seek, find and not give up.”

Junior Lieutenant Maleshkin, an unshooted graduate of the school, commands the crew of the self-propelled gun SU-100. All the subordinates of Maleshkin are older than him and much more experienced. The authority of the young commander is tested at every step, he quarrels, then reconciles with his crew.