People living at a seashore town are frightened by reports of an unknown creature called "the sea devil". Nobody knows what it is, but it's really the son of doctor Salvator. The doctor performed surgery on his son and now young Ichtiander can live under water. This gives him certain advantages, but also creates a lot of problems.
The film is based on the popular science-fiction novel “Professor Dowell’s Head” by Russian writer Alexander Belyaev. For many years, a prominent scientist had been striving to solve the problem of immortality. He shared the results of his daring and risky experiments with his colleagues. But one day something happened that no one could have foreseen: the professor had disappeared. The best student and follower of Professor Dowell tries to use the results of his scientific research for criminal purposes. It looks like he is going to succeed…
At the beginning of the last century, young Lillian, engineer Carter and policeman Simmons meet on an ocean liner, tracking Carter down to arrest him on murder charges. Suddenly , all three find themselves on lifebuoys in the open ocean ....
St. Petersburg, 1900. There is a war between the land and the underwater world: at night, the Sea Devil attacks ships in the Gulf of Finland, and the inventor and industrialist Maxim Naydenov hunts for a semi-mythical monster. Searches Maxim with the mysterious Gala, a girl from the coast, but the hero does not yet know that his lover is not what she seems at first glance, and in this battle she may not be on his side...
Soviet geologist Viktor Klimenko with his guide and assistant, Yakut Nikol, fall into the trap of enterprising businessmen. They are going to conserve all the air in order to sell it monopoly, profit and dictate their will to the rest of the world.
