The story begins in the fictional city of Revsk (the prototype of Revsk, according to the author of the book, was the city of Snovsk) during the Civil War in Russia (1921). The main character, a teenager Misha Polyakov, who came from Moscow for the summer, due to a coincidence, becomes the owner of an officer's dagger of the 18th century without a knife. The former owner of the dagger, a sailor officer, was killed in 1916 a few minutes before the explosion of the battleship Empress Maria. Some secret is connected with the dagger - an encrypted letter is hidden in its handle. The key to the cipher of the dagger is in a sheath, and the sheath is in the White Guard bandit Nikitsky, who served on the battleship with the deceased owner of the dagger. Nikitsky is hunting for a dagger.