In the peaceful kingdom of Hetvia, Count Dakkar (Lionel Barrymore) prepares two high-tech submarines to explore the ocean depths. Jealous of Hetvia's bounty and eager for war, the evil Baron Hubert Falon (Montagu Love) attempts to steal the two submarines for use as weapons. An undersea chase ensues, which leads the two rivals, along with Dakkar's daughter, Sonia (Jane Daly), and her lover, Nikolai Roget (Lloyd Hughes), into a secret world ruled by a race of strange aquatic creatures.