[The following plot summary is based on the English-language version of this film, Devil Monster ; character names refer to that version.] José Francisco has been missing at sea for six years. His mother sees a newspaper story that suggests there may be survivors of the wreck on a distant island. She asks Robert, who has fallen in love with José's girl friend Louise, to check the islands when he is on a tuna fishing expedition with his father. Louise holds out hope that José is still alive. Robert and his father find José on one of the islands, where he has gained the natives' respect as a skilled fisherman. He is in love with the local chief's daughter and unwilling to return to civilization, so Robert, his father and the crew members abduct José. He is assigned to help look for tuna, and he eventually takes them to a productive area, where they encounter the "devil monster," a giant manta ray. Robert is knocked overboard, but José goes to his rescue even though he suspects that Robert wishes to marry Louise. In the rescue, José loses an arm. The boat returns to harbor, and José is reunited with his mother and girl friend.