Two girlfriends—Asya and Mathilda—learn that a ghost has taken up residence in the neighboring apartment: a musical poltergeist. A closed piano… starts playing classical music on its own! Wanting to figure everything out, the girls sneak into the empty apartment. But they can’t find the spirit, because at that very moment the house is being invaded by the real, no-fooling burglars… The girls decided to involve their friends—Mitya and Kostya. In Asya and Mathilda’s opinion, they are reliable guys and will definitely help them uncover the secret of the musical ghost and “identify” the intruders. And so the detective office “Quartet” begins its first investigation!