1930s. During naval maneuvers, three submarines must "attack" the battleship. During a training battle, a fourth periscope appears on the horizon. The battleship is in danger if the unknown submarine is enemy. However, the fourth periscope may also belong to the Soviet submarine Sprut, which has gone off course, which, according to the plan, should not be in this place. "Otvazhnyy" gives call signs, but does not receive a response. And then the commander of the destroyer Vladimir Krainev gives the order to attack the object with depth charges.