Although atom bombs have been dropped on Hiroshima and Nagasaki, a group of German scientists still labors, deep in the Harz mountains of Thuringia, Germany, to fulfill Hitler's dream of supremacy. They intend to explode their own atom bombs, via remote radio detonation, in major cities throughout the world. In New York, German-born Professor Gustave Heinrich Kleinheldt, who is a member of the U.S. atomic team but is secretly working for the Nazis, tells Larry Cameron, the Secret Service agent assigned to guard him, that he is leaving on a fishing trip with his driver Mannfred. Larry decides to follow them and sees their car go off a mountain road. When Larry pulls Kleinheldt from the wreckage, Mannfred shoots Kleinheldt, who as he dies, tells Larry to get to "Kyffhauser." Larry learns that Kleinheldt became a naturalized citizen five years earlier and that Mannfred, who was also naturalized, had worked for him for fifteen years. While Mannfred is being interrogated by the FBI, he takes a poison capsule and dies without revealing anything significant. However, by examining fingerprints taken from Kleinheldt's body, Larry establishes that it was not the real Kleinheldt in the car but a double. When it is discovered that the genuine Kleinheldt has flown to Germany, Larry is assigned to follow him. In the Harz Mountains, Larry meets Greta Holvig, a reporter for the Scandinavian Press Syndicate, who tells him that she has read about his involvement in the Kleinheldt case. Using the password "Rendezvous 24," Larry meets up with George Timothy, a British agent, who is working undercover as a waiter at the inn where Larry and Greta are staying. Greta is actually a German agent sent to kill Larry, but her initial attempt on his life is thwarted. While Larry and George try to decipher a possible clue to the location of the scientists, Greta reports to her contact, Dr. Heligmann, who sends agents Becker and Ernst to trap them. Larry and George trace a radio signal, which has been interfering with their radio communication with London, to a remote cabin, which Becker and Ernst have booby-trapped. However, Larry spots the wires, and discovers through another clue that Kyffhauser is in Russian occupied territory. Later, Larry is caught by one of Heligmann's men and a false message is sent to George asking him to join Larry at Heligmann's farmhouse. London intelligence advises George that Greta is a German agent so that when she brings the phony message, George knows that it is a trap. Heligmann learns from scientist Zarek that Kleinheldt has perfected the remote, radio-controlled explosion of an atomic bomb and proceeds with plans to explode one in Paris that evening. As George and Greta reach the farmhouse, George draws a gun on Greta and tells her that the body of the real Greta Holvig was found two days earlier. George pushes her into the room where Larry is being held prisoner and, in the ensuing confusion, Larry grabs one of his guard's guns. A gunfight errupts and Greta is hit. As George and Larry chase Heligmann and Zarek into a tunnel beneath the farmhouse, Larry shoots Zarek and takes Heligmann prisoner. A small electric car carries Larry, George and Heligmann directly into a laboratory in the Kyffhauser mountain, where Kleinheldt is preparing to detonate the bomb. With only seconds to spare, Larry shoots Kleinheldt and Paris is saved.