In the late 1930's, Marine lieutenants Tom Grayson and Frank Corby are stationed in Shanghai when they receive orders to march their men through the jungle to the aid of Americans trapped in Manchuria. All goes well until they reach Linchuria, where, at the fort of a Chinese ally, they discover that all of the occupants have been mysteriously killed. As the Marines investigate, the building is hit by a torpedo that generates a huge quantity of electricity, and all of the men are killed except Tom and Frank. Tom, who was in command, is brought before a tribunal, and during the court-martial proceedings, he receives a letter from Lin Wing, the Linchurian consul. When the prosecutor telephones Wing about the letter, the nervous consul confides that he has information about the weapon used in China and about The Lightning, a diabolical mastermind seeking to control the world. Wing is killed before he can reveal more information, however, and Tom's trial is postponed so that he can investigate the murder. Tom and Frank find half-burned papers in Wing's fireplace and take them to the home of Warfield, a prominent scientist and old friend of Tom's father, Col. Grayson. Warfield is not surprised by Tom's description of The Lightning's electrical torpedo and states that such a weapon is possible. As the group, which includes Warfield's daughter Janet, examines the papers, a stranger peers through the window. Tom and Frank chase the man and his companion, and while one of the men escapes, the other is killed. The Marines then learn that the S.S. Rockingham is in danger. Soon after, the Rockingham is hit by one of The Lightning's torpedos, and its cargo, a gold shipment, is stolen. Col. Grayson's research work with Warfield intensifies after the incident, and one night, the laboratory is targeted by The Lightning, and Grayson is killed. Crenshaw, a scientist who was working on the project, admits to Tom the next day that he left the lab out of fear but now wants to avenge Grayson's death. Through Warfield's analysis of a shell fragment, Tom discovers that the casings for The Lightning's weapons are being made at the Atlas Steel Co. After several encounters with The Lightning's henchmen, Tom locates the shipping information for the casings, which leads the Marines to Gehorda Island in the tropics. There, Tom and Frank find a schooner transporting more casings to The Lightning's secret hideout. During their search of the schooner, Tom and Frank learn that the torpedoes are guided by a gyroscopic control. Their search is cut short by the return of the crew, and the ship is set aflame and sinks during the ensuing brawl. Tom and Frank escape, and later dive at the site of the sunken ship to find the gyroscope. Despite interference from more henchmen and a shark, the Marines find the gyroscope and take it to Warfield's home laboratory. Before Warfield can analyze it, however, The Lightning sends word that he has kidnapped Janet. At the hangar housing the "Wing," The Lightning's ultra-modern plane, Janet is astonished to learn the masked villain's identity, and he vows never to release her. Janet is taken to Gehorda and is seen there by Tom and Frank, who have found The Lightning's cave hideout on the island, but are unable to rescue her. After vanquishing more of The Lightning's men, Tom and Frank return to the mainland, where Crenshaw arranges a meeting to test his new ray machine, which can destroy the electrical torpedoes before they are launched from the Wing. Tom and Frank accompany their commander, General White, and Warfield to the conference room, where they debate the possibility of The Lightning having a spy in their midst. As the men talk, several gang members pump carbon monoxide into the locked room, but Tom's quick action saves them from death. Tom then discovers a clue on the floor, and the next day, calls a meeting at Warfield's home, at which White, Crenshaw, Benson the butler and Sam Hedges, the gardener, are also present. Janet, who has been rescued by Tom, appears and is about to reveal The Lightning's identity when a bolt of electricity is shot at her. Hidden behind a mirror, Janet is protected from the bolt's charge, and the culprit is revealed to be Warfield. Tom then explains that he deduced The Lightning's identity upon finding a tiny, hand-held gas mask on the floor near Warfield's chair in the conference room after the carbon monoxide attack. Warfield then escapes to the Wing, but as he prepares to launch a torpedo at the house, Crenshaw proves the effectiveness of his ray by blowing up the Wing and its evil occupants.