Soviet scientists discovered on the bottom of the Black Sea an amazing find with the help of an echolocation: an apparently man-made structure, probably a whole city. Belka and Strelka, the first astronauts, are sent to investigate this mysterious object. As deep divers, they descend to the depth and discover the beautiful, high-tech and culturally developed city which is inhabited by animals who are extinct on the surface of the Earth. Preforming another heroic mission, space dog, as well as their friends, help to prevent the destruction of unique animals and an underwater city.