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...