Submarine Alert

During World War II, FBI agent Fleming believes German spies are tipped off by routine radio activity and proposes a ruse to identify them. He suggests firing all local radio specialists, hoping unemployed workers will seek any available job and reveal themselves. The plan sets the espionage hunt in motion and frames the conflict around radio technology.

Lew Deerhold, a Canadian citizen, cares for his war orphaned niece Tina who needs a brain operation. He takes desperate steps to earn money and applies for work at a local radio repair shop. There he meets Ann Patterson, an FBI agent secretly following him, foreshadowing the larger conspiracy.

In Lew's apartment, Dr. Arthur Huneker and his assistant Vincent Bela present Lew with a job to repair a stolen, top-secret radio transmitter. Ann Patterson continues to shadow Lew as part of her FBI assignment. Freddie Grayson, another agent, searches Lew's place and is shot, yet he manages to warn that the doctor possesses the transmitter.

Lew confronts Huneker as he meets with Japanese Commander Toyo to recruit Lew into the Nazi operation. He pretends to cooperate while secretly plotting resistance. The tense exchange compounds the danger and deepens the espionage plot.

Huneker and Toyo begin torturing the owner of Bambridge Shipping to coerce cooperation. Lew is ordered to kill Bambridge, who is really American agent Captain Hargas. Lew escapes the scene with the transmitter codes, slipping out to continue the mission.

Lew and Ann join forces to thwart the Nazi plot, but Huneker captures them and locks them in a steam room. Facing a deadly heat, Lew improvises a plan and begins to work on a transmitter to signal for help. The moment tightens the bond between the two protagonists amid danger.

From the steam room, Lew devises a makeshift transmitter and sends an SOS. A young boy picks up the distress signal and his father immediately calls the FBI. The inadvertent transmission becomes the critical link that triggers a rescue operation.

Agents rush to Lew and Ann's location and arrest Huneker and his accomplices. Fleming coordinates the crackdown, eliminating the immediate Nazi threat in the area. The arrests restore some stability and reveal the extent of the network.

Agent Fleming contacts a bomber squadron that targets the Japanese submarine lurking off the California coast. The decisive air strike protects shipping lanes and demonstrates effective cooperation between intelligence and military power. The action underscores the broader war effort of the home front.

With Tina recovered from her brain operation, she is present for the victory moment and relief. The success brightens the atmosphere and confirms the personal stakes behind the espionage mission. The family reunion scene emphasizes the human cost and reward of the war effort.

In the film's closing arc, Lew is granted American citizenship and serves as a private in the US Army. Ann attends the moment, highlighting their ongoing partnership in service to the country. The culmination ties Lew's personal triumph to his renewed national allegiance.