A schooner disappears at sea without a trace. Years later, evidence of possible survivors prompts the mother of the schooner's mate Jose to hire a tuna boat to investigate. They discover the lad living happily on a South Seas island, and, when he refuses to leave with them, they abduct him. However, Jose gets revenge by leading the ship into the lair of a mysterious giant manta ray.

A movie serial in 12 chapters: The famous comic strip character is on a mission to protect a secret tunnel passage between China and India.

The Sea Fiend, aka Devil Monster, is an American adventure film directed by S. Edwin Graham and it was shown in Great Britain as The Sea Fiend in 1938. An edited version of the film was released in 1946 as Devil Monster, a low-budget South Seas drama spiced up with stock footage inserts including half-dressed native girls that were also featured in the film's trailer.