On the day Coast Guard lieutenant Terry Kent receives a distinguished service medal, he is ordered to report to Commander Boyle at once. Terry bids farewell to his brother Jim, a fellow Coast Guard officer, his girl friend, reporter Jean Norman, and Jean's photographer, Snapper McGee, then reports to Boyle. Boyle tells Terry that a suspicious-looking yacht has been spotted outside the twelve-mile limit and orders him to investigate. Despite his best efforts, Terry loses the yacht in the fog, and its passenger disembarks at an island, where a secret meeting is being held. Attending the meeting are representatives of the country Morovania, who are being shown a motion picture demonstration of a new radioactive gas made from the mineral arnatite. The gas destroys everything except glass and is so powerful that only a few small bombs of it are needed to eradicate an entire city. The representatives are amazed and declare that the gas seems like the work of the supposedly dead scientist Boroff, a man as evil as he is brilliant. The men order the gas and are told that boxes of arnatite will be shipped to their country the next day. After they leave, the demonstrator congratulates Boroff, who is alive and is indeed the gas's inventor. The next day, however, a storm almost capsizes Boroff's ship, the S.S. Carfax , as it tries to leave with his cargo. The Coast Guard is called, and when Jean and Snapper, who have tagged along, recognize Boroff, the scientist flees. Jim pursues him and is killed, after which Terry vows revenge. The Coast Guard anchors the Carfax , and Boroff sends his servant Thorg, a giant whom has he has cruelly mistreated, to sever the cables. Snapper and Jean, who are aboard the ship, alert Terry, and although Thorg escapes during the ensuing mêlée, the ship is sunk. Terry, who retrieved Boroff's notes before the ship sank, takes the information to Dick, Jean's chemist brother. Boroff, meanwhile, charters a scow and sends Thorg to dive to the Carfax 's hold and retrieve the boxes of arnatite. Terry finds out about the covert operation, but the giant escapes again after another battle. While Boroff is manufacturing the gas in a huge cave inside a nearby island, Dick is nearly killed while duplicating the gas. When he later visits Dick in the hospital, Terry learns that an electroscope can detect arnatite, and so borrows the hospital's equipment to track Boroff to his hiding place. Terry is captured and held hostage in the cave, and Boroff notifies Boyle that he will kill Terry unless he is allowed to leave with his cargo. Knowing that Terry would rather be sacrificed than let Boroff escape, Boyle orders his men to attack the island. As the shore troops are approaching the cave, Boroff releases the gas, which disintegrates the ground around them. The men are about to be killed when Jean flies overhead and sprays the counter-agent that Dick has developed. Terry, who has freed himself, pursues Boroff, while Boyle's men round up the rest of the gang. Before Terry can catch Boroff, however, Thorg, who had been shot by Boroff for letting Terry escape, finally seeks his revenge on his cruel master. Trying to kill the monster, Boroff throws a gas bomb at him, but the gas eats through their boat and both are drowned. Later, members of the Coast Guard throw rice at Terry and Jean as they leave the church after their wedding.