This is a last of 3 movies about 3 friends (the other two are "Kortik" and "Bronzovaya Ptitsa"). The gang has grown up and in this movie they discover and help to stop criminal activity on the factory and in their neighborhood.

Based on the book of Viktor Dragunsky "Deniskin's Stories". Once upon a time there lived in Moscow the 1950's an October school student Deniska Korablyov. Every day he was terribly busy with the most ordinary children's activities: he did not prepare lessons, quarreled and put up with classmates, helped around the house and dabbled. He also planned a real murder, crossed paths with the criminal Fantômas, exposed the adult liars and thieves, participated in the selfless rescue of man...