During World War II, American journalist David March defects to Nazi Germany, and is soon given a job broadcasting propaganda to the West. Capt. Elm is aware of an American spy ring called Blue Light, and that of its 18 original members, 17 have been killed. He sets out to catch the man he believes is the last member of that group: David March.
David tries to convince top Nazi officials that he is a loyal Nazi so he can be assigned to a top-secret weapons base in Grossmunchen. Unknown to him, a British officer has sent a team of commandos to kill the man he believes to be a dangerous traitor: David March.
David finally gets the assignment he has been working towards: the top-secret weapons base at Grossmunchen. He discovers that the heavily guarded and fortified base is 200 feet underground and the only thing more difficult than getting into it is getting out of it. Nevertheless, he must find a way to destroy it.
David has convinced a female scientist working at the base to help him blow it up, but she is starting to have seconds thoughts when she realizes that she will be causing the deaths of all her friends and colleagues inside.
David pays a visit to his younger brother, who is in a POW camp. He discovers that his brother had lied about his age to join the army and show everyone that, unlike his older brother, he wasn't a traitor.
A Soviet agent finds Nazi plans for heavy-water experiments but is captured, so the Allies order March to get the information before Nazi interrogation and execution, but the Soviet shows he's loyal to Russia, not the Allies.
The Nazis capture a Navy commander with secret information in Scotland, bring him to Germany, and have March try to win him over, but the Allies bomb the building and it's a race against time before air runs out.
The Allies need to have Hitler's invasion of England, called Operation Sea Lion, postponed. David, knowing how much faith Adolf Hitler puts in astrology, hatches a plan to get Hitler's astrologer to convince him that "the stars" demand that the invasion be delayed.
Capt. Elm, the Nazi officer who knows that David is a double agent, escapes from a British POW camp and makes his way to Berlin, determined to prove to his superiors that David, and not he, is the real traitor--and to kill him.
David is forced to kill a Nazi courier who has the plans for every rocket installation in Norway. A young Nazi boy, 11 years old, sees David kill the man. David must choose between killing the young boy or being exposed as a double agent.
As a test of his loyalty, the Germans send David into battle posing as an American soldier. His assignment is to learn the plans of the Allied armies in Italy. To ensure that he completes his task, they send a Gestapo agent along with him to keep on eye on him.
A wealthy German aristocrat lets the Allies know he wants to help overthrow Hitler. They send an agent to meet with the man, but the Nazis capture and execute him. David is assigned to take the agent's place.
David and Suzanne are in France to do a radio show when they discover that the Germans have broken the underground's code and are planning an ambush for a coming operation by Allied commandos. David and Suzanne have to find a way to warn the commandos about the trap without placing themselves under suspicion.