Sign of the Green Arrow

A sailor standing watch aboard a scientific schooner off the coast of Haiti observes a series of green lights in the shape of an arrow coming from the island's interior. I time he figures out that the lights are some kind of code. It was a time prior to World War II and since no other surface vessels were in the area, he deduces the lights are being flashed by spies aimed at a submarine. Time proves his conclusions to be correct and he becomes determined to locate the source of the mysterious green arrow lights. When the daughter of an island scientist is kidnapped, the sailor enlists members of the crew and island natives to discover the nest of spies and rescue the girl who is being held captive aboard an enemy submarine.