When Captain John Winslow of the U.S.S. Carolina is notified by the Secretary of the Navy that his cruiser will be retrofitted with a new firing control gear, a device that is predicted to revolutionize naval warfare, he is put in charge of overseeing the operation. Orders are given to install the new machinery immediately in order to prevent the World Electric company, its manufacturer, from selling it to a foreign power. The gear, which had been due to arrive at the port for loading onto the ship some ten days earlier, mysteriously disappears, but is soon found by intelligence officers being transported across the Mexican border. When the device finally arrives, the secrecy surrounding its installation arouses the curiosity of many, including reporter Walter Drake, who is anxious to break the story on the firing gear. Lieutenant Tom Randolph, the fire control officer in charge of the new device, and Captain Winslow welcome visitors Al Duval, an engineer from the World Electric Company, and Victor Hanson, a civilian inspector from the Navy Department, aboard their ship while the device is put into place. Meanwhile, on shore, Toots Timmons tells her sweetheart, sailor Spud Burke, that she wants to marry him without further delay. At the same time, Betty Lansing boards the ship and runs into Tom, who had proposed marriage to her some time ago and then spurned her, and she asks him to leave the Navy and take a job from her father, who has offered to put him in second command of a dam building project in Ecuador. Tom rejects the offer and reminds Betty of his love for the Navy. As the new fire-control gear is lifted onto the ship by a large crane, the payload is dropped in an apparent accident. Later, however, it is determined that the broken cable responsible for the mishap was deliberately severed in an attempt to sabotage the delivery. Dignitaries representing the Oriental Consul board the ship for a tour, followed by a man calling himself Harry Jeffries from the Gyroscope Company, who offers Tom $25,000 to sabotage the new device. Confusion ensues aboard the cruiser following the mysterious murder of Duval during a gun salute, and the captain seals off the ship. Suspicion is cast upon a number of people, including the Oriental consul's secretary, who cannot be accounted for; a civilian woman who is found toting a gun; and Mr. Jeffries, who is found hiding in a turret. No sooner does the investigation into the murder begin, than another man, the chief electrician, is murdered. Later, Drake is caught trying to flee from the ship, but Tom does not believe he nor any of the other suspects are killers. While Spud and his friend, Mac O'Neill, vie for the affections of Toots, a plan to lure the murderer into a trap is implemented by the captain. Soon, the trail of evidence leads Tom to the powder magazine, where he finds the crazed Victor holding a flame to the ship's explosives and threatening to blow up the ship. Hanson tells Tom that the World Electric Company stole his idea for the firing device and that he wants revenge. A fight ensues between the two, and the magazine is flooded in order to prevent the explosives from igniting. The men come precariously close to drowning in the magazine until the door is unlocked and Hanson is captured. With the murderer caught and the new device in place, the Carolina is finally ready to set sail.