The film tells about an underwater expedition in the Sea of Japan off Monneron Island, located in the southwest of Sakhalin Island. Monneron Island. Rocks. Surf. Expedition camp. Filming is carried out by scientists, film enthusiasts, diving masters: V. Suetin, A. Migdal, O. Severtseva, A. Kalmanson. Thickets of seaweed, jellyfish, sea urchins, starfish, sea cucumbers, crabs, octopus, seals, ascidians.