The Great Patriotic War. The Soviet submarine T-9, passing through the minefields of the North Sea, penetrates the enemy port. Sailors drown enemy transport ships, but soon find out that they are late - enemy troops and weapons are already unloaded and sent to the front. Captain Kostrov finds himself in a situation where a combat mission is almost impossible, but he must fulfill it - for him it is a matter of honor or dishonor.