They Came to Blow Up America

In an American courtroom, six of eight would-be German saboteurs are sentenced to death. An FBI agent complains to his boss Craig about the two who remain. Craig explains that one of the men is already in prison, which prompts a flashback that recounts Carl Steelman's undercover mission.

Carl Steelman stuns his German-born parents by telling them he is a member of the German American Bund. His father, loyal to America, is incensed by the admission. The scene sets up Carl's double life and the stakes of his undercover work.

At a Bund meeting, Reiter reveals he has been called back to Germany to be trained as a saboteur. Police break up the meeting; Reiter draws a gun and is shot dead.

Carl meets with Craig and is ordered to impersonate Reiter and infiltrate the saboteur school. He begins planning his undercover entrance as the mission unfolds.

In Hamburg, Carl becomes friendly with Helga Lorenz, a local woman who will become entangled in his undercover work. The relationship complicates his mission as the story moves through his dual roles.

Colonel Taeger informs Carl that Helga is suspected of involvement with the German Underground and orders him to continue seeing her to gather incriminating evidence. Carl's loyalty to the mission strains his personal ties.

Carl finds anti-Nazi leaflets and warns Helga, but a spy witnesses. He is forced to denounce her to Taeger, and when she resists talking, Taeger orders her sent to a concentration camp. Carl subsequently intercepts the transport and rescues her.

Reiter's wife shows up at his hotel, and to prevent betrayal, Carl lies that Reiter has been captured in America and asks for time to explain himself. He later informs Colonel Taeger that his 'wife' has lost her mind, resulting in the wife being sent to an asylum.

When Carl's father becomes seriously ill, Craig informs Julius about Carl's real mission but warns him not to tell anyone, not even his wife. Julius cannot bear the secrecy and tells his close friend, Dr. Herman Holger, about Carl's mission.

Carl's excellent performance at the sabotage school earns him command of the first group of saboteurs to be sent to America after the U.S. enters the war. He is placed in charge along with three others, preparing for deployment.

Carl and three others board a U-boat as part of the mission, marking a critical phase of their undercover operation. The journey underscores the danger and stakes involved.

Taeger is notified that a Carl Steelman is an American agent and travels to Frau Reiter, who confirms the connection. Taeger shoots her to cover up his mistake and then sends an urgent message to the U-boat captain.

The submarine is attacked by American bombers but escapes unharmed, and the saboteurs depart in a life raft. A bomb Carl left aboard detonates, sending the submerged U-boat to the bottom.

Ashore, the four men are spotted on the beach and arrested. Craig instructs Carl to maintain his cover and turn state's evidence against the others, including the four saboteurs sent to Florida.

Carl returns home to see his parents and Dr. Holger. He informs Holger that he has a list of German agents in America, with the doctor listed as agent number eight, and he takes Holger away.