Corvette Soobrazitelny has reached Kronstadt, near St. Petersburg. There, its crew is begins getting ready for the parade. They face the huge task of cleaning and painting the whole ship, until it shines. In the meantime, the submarine Magnitogorsk has been placed in charge of monitoring the dive of another submarine. It’s then that its captain, Vladimir Nagornov, spots a civilian ship in the vicinity of where the submarine Vyborg is about to surface, which can be very dangerous.