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The heroes of the story - a fragile young violinist Olenka and her friends Anton, Sasha and Sergey - while on holiday in a camp on the Black Sea coast accidentally learn about the secret of the treasure of the famous Milady, the heroine of the novel about the Three Musketeers, hidden nearby. At night, the friends secretly go in search of the treasure, but find themselves in a whirlwind of exciting adventures. They will have to make a dangerous journey to the smugglers' cave, as well as engage in a real fight with the thieves of museum valuables.

Who will win in the eternal battle between evil and good - between cunning and hatred, or sincerity and friendship?