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On a Tokyo street corner, at noon, U.S. Army intelligence agent Jim Carter, Eastern European exile Steffi Novak and Japanese cabinet minister Matsudo are waiting together when a bomb explodes nearby.  Jim then recalls the events of the past two weeks that led to the blast: Jim arrives in Tokyo and learns from Colonel Groves that his assignment is to pose as a reporter in order to gather information about Communist activities.  Specifically, he is to renew his friendship with his college roommate, Matsudo's son Taro, a known Communist sympathizer.

At the Tokyo Correspondence Club, Jim meets the resourceful Steffi, who offers to help him with his story.  That night, Steffi and Joe, a Japanese agent posing as a taxi driver, accompany Jim on a tour of Tokyo night life.  After Joe drives Jim and Steffi to a club that Taro is known to frequent, Jim pretends to be drunk and starts singing his college fight song.  As hoped, the song draws Taro out, but as soon as he sees Jim, he flees.

Later, at Steffi's place, Jim notices a photograph of a young woman taken in North Korea, which Steffi admits is of her sister Christine.  Jim secretly instructs Joe to copy the photograph so that Groves can obtain information about Christine.  The next day, Jim visits Matsudo, who tells him that during the war, Taro was indoctrinated as a kamikaze pilot and felt betrayed when the Japanese surrendered.  A confused Taro fell in with the Communists and broke his engagement to his sweetheart Naniko.

Upon returning to the Correspondence Club, Jim receives a telegram directing him to go alone to the festival in Enoshima.  After eluding a man who is tailing him, Jim arrives at the festival and is approached by Taro.  Later, in his room, Jim is beaten by some Japanese men and told to heed Taro's warning to mind his own business.  The next day, Jim informs Groves that the men got into his room with the help of some Russians, whose staff building is next to the Club.  Jim also tells Groves that, at the festival, Taro acted cold except when Jim mentioned Naniko.

As arranged by Steffi, Jim then dines with businessman Oyama at a posh hotel.  During the meal, Jim receives a phone call from a man claiming to be police inspector Tabuchi, who orders him to report to the Intelligence Office.  After the caller hangs up, Joe warns Jim that if he does not answer Oyama's questions correctly, Oyama will poison him.  Back at the table, Jim casually tells Oyama that the police have mistaken him for an intelligence agent and, while nervously accepting Oyama's proffered dishes, maintains his pose as a writer.  Jim survives the meal, and the next morning, is given a note advising him that he can find Naniko, an actress, at the Imperial Theater.  There, Naniko admits that she still loves Taro and offers to help Jim.  As she leaves the theater, however, Naniko is kidnapped.  When undercover police give chase, the kidnappers throw Naniko from their car and flee.

Later, Taro sneaks in to see Naniko in her hospital room, and although she and Jim insist that Taro's comrades were behind the abduction, he refuses to believe them.  Jim then deduces that Steffi sent him the message regarding Naniko and goes to her place to arrest her.  After Jim shows her an intelligence report stating that her sister was murdered six months before in North Korea, Steffi confesses that she has been helping Oyama, the Communists' leader, in order to get Christine out of Korea.  Oyama had promised to intercede on her behalf, while using the sisters' correspondence to convey coded military information.  Jim convinces the revenge-hungry Steffi not to confront Oyama but try to discover where his secret papers are kept.  Oyama catches Steffi in his office and becomes suspicious, but Steffi acts nonchalant and starts to leave.  When Taro and his cohorts arrive for a meeting, however, Steffi lingers by the door to eavesdrop.

After Steffi overhears Oyama ordering Taro to instigate a labor strike as a way to distract Japan from the war in Korea, she relays the news to Jim and Matsudo.  That night, Taro tries to foment unrest among some railroad workers, but Matsudo delivers his own speech, which causes a brawl to erupt.  Taro runs away and, while stumbling in the dark, ponders his current state.

In the morning, Taro goes to Naniko's bedside and begs her forgiveness.  Naniko is strangely silent and Taro realizes she is dead.  Taro's comrades then drag him to Oyama's office, where Oyama informs him that he has sent forged notes to Jim, Steffi and Matsudo, asking them to meet Taro on the corner outside his office.  Oyama also reveals that a bomb is about to go off and a forged confessional note from Taro has been planted.  To foil Oyama's plan, Taro rips off Oyama's suit pocket and, while clutching the incriminating piece of cloth, jumps from the office window.  Seconds before the bomb explodes, Jim, Steffi and Matsudo hear the commotion and rush toward Taro's crushed body.  Taro's sacrifice saves their lives, and when Oyama attempts to sell his secrets to Jim to avoid arrest, he is stabbed to death by his right-hand man.