Frank Raymond, grandson of the original Invisible Man, still has the old formula but considers it too dangerous to use, even when Axis agents try to get it. But Pearl Harbor brings him to volunteer his own services as an invisible agent in Germany. Though a bit cold (clothes aren't invisible), his adventures are more comedy than thriller (with occasional grim reminders) as he makes fools of Nazi officials and romances a luscious double agent, in search of Hitler's secret plan...

Foreign spies Baron Ikito and Conrad Stauffer go to the print shop of Frank Raymond, whose real name is Frank Griffin, in an attempt to buy his grandfather's infamous formula for invisibility. When Frank refuses, they threaten to cut off his fingers, but he manages to escape. Frank then meets with government agent John Gardiner, who asks him to turn over the secret formula to the United States government. Frank tells Gardiner that his grandfather's invention is too hazardous, but he changes his mind when America enters World War II. After he insists that he be the only person to use the dangerous drug, Frank is parachuted into Berlin in order to discover Germany's plans to sabotage the American munitions industry. Using his cloak of invisibility, he eludes a German patrol and meets with his contact, mortician Dr. Arnold Schmidt. Schmidt sends him to the home of double agent Maria Sorenson, who is dining that night with Gestapo officer Karl Heiser. Heiser tells Maria that Hitler has ordered a secret attack on the United States, but does not tell her the exact date after the invisible, and slightly drunk, Frank ruins the Gestapo man's plans for a romantic tryst. Later that night, Stauffer and Ikito return to Berlin, and Stauffer questions Heiser about the missing parachutist and Heiser's activities with Maria. The two Gestapo officers then go to Maria's home, where Stauffer places Heiser under arrest for trying to steal his job and girl friend. Suspecting that Maria is harboring Frank, Stauffer then announces that he has, in his office, the secret plans for the invasion of America. The inexperienced American spy falls for the bait and becomes trapped in Stauffer's office, where, despite his invisibility, he is questioned by the Gestapo officer. Frank manages to escape, however, by setting the office on fire, and takes with him a list of every German and Japanese agent in America. After giving the information to Schmidt, Frank goes to the German prison holding Heiser and agrees to release the condemned man if he reveals everything about the planned German attack on the United States. Heiser agrees and states that a suicide plane squadron is leaving Germany for New York that night. Meanwhile, Schmidt is arrested and questioned by Stauffer, so when Frank stops at the mortician's home, he finds only Maria. After Frank accuses her of being a German spy, they are both captured by Ikito and his Japanese agents. Ikito then makes plans to take Frank to Japan, but Frank and Maria flee when Stauffer and his men raid the Japanese embassy. Realizing that he has failed in his mission, and thus dishonored his country, Ikito murders Stauffer, then commits hara-kiri. Frank and Maria make their escape in a German plane, but first bomb the airfield holding the planes bound for New York. Later, their plane is shot down over England, but they manage to parachute to safety. Frank is then taken to an English hospital, where his invisibility wears off and he learns that Maria is truly a British agent whose real name is Goodrich.