What secret have the inhabitants of a huge St. Petersburg communal apartment kept for many years? Why are they so afraid to open the door to the back staircase, along which servants once climbed and led away “enemies of the people” in the morning? The curiosity of Maria, who decided to go down the dark stairs to the street, only added fear to the residents of the communal apartment - a mysterious lady in black began to come to the apartment. Maria and her friend Ilya had to find out why the lady was coming, what she was looking for in the rooms and why she was angry with the residents?