The action takes place during the Second World War in the battle zone over the Black Sea coast. Soviet sailors track down an enemy submarine. Meanwhile, the commander of the Sea Hunter, along with one of the sailors, is not returning from intelligence. Young patriots Katya and Lida report to the boat about the signal of unknown friends who noticed a submarine. Thanks to this, Soviet sailors drown an enemy boat, and the brave reconnaissance Katya, who knows the mountain paths of the coast well, discovers a Sea Hunter commander and a seriously wounded sailor in one of the caves ..