Mystery Liner (1934)

Captain Holling is relieved of command of his ship after he suffers a nervous breakdown. His replacement, Captain Downey, takes over the liner just as it is about to be used for an experiment in remote control. Professor Grimson has devised a system for controlling the ship from a laboratory on land. But as Grimson demonstrates the system and the special component that makes it work, a rival group is listening in, hoping to use the device for its own purposes. Shortly after the demonstration, the professor is attacked and fatally injured. Major Pope comes on board to investigate the attack, and he decides to come along on the planned trip. Soon the ship is full of passengers and crew--and at least two of the ship's occupants are really enemy agents.