Two girlfriends—Asya and Matilda—find out that a ghost has appeared in the neighboring apartment: a musical poltergeist. A closed grand piano… starts playing classical music by itself! Wanting to figure everything out, the girls sneak into the empty apartment. But they can’t find the ghost, because at that very moment the house is about to be broken into by the real, true thieves… The girls decide to involve their friends—Mitya and Kostya. In Asya and Matilda’s opinion, they’re reliable guys and will definitely help them uncover the mystery of the musical haunting and “catch” the intruders. So the “Quartet” detective bureau begins its first investigation!