Russia, the second half of the 19th century. Adventure seekers can’t stop being haunted by the secrets of ancient burial mounds. Legend says that Khan Mamai sleeps beneath one of them and keeps his peace with a horse made of pure gold. The son of a wealthy landowner who decides to go looking for the legendary horse needs reliable servants—hardworking and not superstitious. That’s why he takes with him his father’s former serf, a young man named Yegor. After all, he wasn’t afraid to invite a mavka to marry him. Only the young gentleman doesn’t know that, as a thank-you, the mermaid gives the would-be groom advice: don’t catch the golden horse.