The day before the ocean liner Guthrie is to sail on an experimental voyage, Captain Holling, who has been suffering from an exotic mental disorder, is declared unfit for duty by shipowner Watson and ordered ashore. Before Chief Mate Downey assumes command, he and First Officer Cliff Rogers learn about the mission and the "S-505," a radio tube device that will enable its inventor, Professor Grimson, to control the movements of the liner from shore. That night, after Grimson is strangled and almost killed, the leader of an enemy government sends an unseen agent to "keep an eye on" another agent already on board the Guthrie . In turn, Watson orders Major Pope, a private detective, to board the ship in order to investigate any suspicious passengers. After ship physician Dr. Howard reveals that Holling was poisoned with a tropical drug, Pope begins to question Downey and Cliff. News of Holling's mysterious appearance in the cabin of elderly Mrs. Plimpton is echoed by the suspicious movements of German passenger Von Kessling. Hours before the S-505 test, Downey is killed as he tries to send a televised message to Watson on shore. As Pope questions Cliff and Von Kessling about Downey's murder, nurse Lila Kane reveals that Downey had stolen a library book about voodoo medicine, implicating him in Holling's poisoning. During the scheduled S-505 demonstration, Grimson extinguishes the ship's lights, and in the darkness, the S-505 is stolen, the ship is immobilized, and Mrs. Plimpton is terrorized by frantic fighting in her cabin. Once the lights are restored, Captain Holling steps out of a secret door in Mrs. Plimpton's cabin and discloses that he and agent Von Kessling were tracking down Major Pope, the real murderous agent, who is then arrested.