From post-war wanderings and evacuation, Lisa and her son Petya return to Moscow and settle in a communal four-room apartment. Their appearance takes by surprise the only inhabitant of the communal apartment, Ilya Nikolin. One day, left alone, Petya enters Nikolin's room, which was unlocked due to forgetfulness, and finds a door to the sea. No one understands whose planet it is and what year it is.
