Don Winslow of the Navy: Chapter 10- Fighting Fathoms Deep

From the land of public domain comics and public domain World War Two live action serials, comes newly minted Commander Don Winslow (the original Commander Marvel), who is returned to the Office of Naval Intelligence from his command of his cruiser to investigate strange events on the Pacific island of Tangita, noticeably a ship being torpedoed. He discovers that there is a ring of saboteurs and enemy agents who are trying to destroy ships carrying supplies to the troops stationed in the islands and sabotage the war effort. Though the US Navy is preparing to build a naval base on Tangita, an unknown foreign power secretly has a subterranean submarine base beneath the island with the goal of preventing the American base from being completed. He sets out with three assistants to find the mastermind behind the activities.

Learn more about the original Commander Marvel, Don Winslow at: https://en.wikipedia.org/wiki/Don_Winslow_of_the_Navy_(comic_strip)
Learn more about this film serial at: https://en.wikipedia.org/wiki/Don_Winslow_of_the_Navy
Learn more about the Commander's second film serial, Don Winslow of the Coast Guard, at: https://en.wikipedia.org/wiki/Don_Winslow_of_the_Coast_Guard