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.

Петроград, 1922 год. Несколько юных пионеров оказываются в центре детективной истории. Им удается выследить опасного преступника, в руках которого — ключ к разгадке тайны старинного морского кортика…