Based on a novel of the same title by Anatoly Rybakov.
Two boys, Misha and Genka, are helping Commissar Polevoy trace Nikitsky, leader of the White bandit gang; years ago, in front of the seamen, Nikitsky killed a naval officer and wanted to take his antique dirk, but Polevoy kept the blade. All this time Nikolsky, who only got the sheath, has been after the dirk.
The boys find out that the dirk is connected with an ancient mystery which can be revealed only with the dirk sheathed. Finally, Nikitsky is seized, and the sheath recovered.
Source : www.lenfilm.ru