Vienna, devastated and recovering from World War II, is divided into four separate zones, each governed by one of the victorious Allies, and a jointly-administered international zone. American pulp Western writer Holly Martins (Joseph Cotten) arrives seeking an old friend, Harry Lime (Orson Welles), who has offered him the opportunity to work with him there.
Arriving at Lime's apartment, Martins discovers that Lime was recently hit and killed by a lorry while crossing the street. Shocked, Martins heads to the cemetery to attend his friend's funeral, where he meets two British Army Royal Military Policemen: Sergeant Paine (Bernard Lee), a fan of Martins's books, and his superior, Major Calloway (Trevor Howard). After the services, Martins accepts an invitation to speak to the members of a local book club, delaying his departure to do so. He is contacted by a friend of Lime's, Baron Kurtz (Ernst Deutsch), who wants to talk about Lime's death. Kurtz relates that he and Popescu (Siegfried Breuer), another friend of Lime's, had picked Lime up after the accident and brought him over to the side of the street, where before dying he had asked them to take care of Martins and Anna, his actress girlfriend. Kurtz mentions the theatre where Anna (Alida Valli) works, but advises that the case is pointless to pursue and best left.
Martins meets Anna after the show. During their conversation, he becomes suspicious and wonders if Lime's death had really been an accident. The porter at Lime's apartment house (Paul Hörbiger) tells Martins that Lime could not possibly have been alive after being hit by the lorry, "not with the way his head was", and adds that he saw a third man helping to carry the body across the street, not just two as Kurtz and Popescu had described. Martins pressures the porter to tell his story to the police, but the man refuses, becoming agitated, and asks Martins to leave.
Martins walks Anna back to her apartment, where they find the police searching her room. The police find a forged passport used to escape the city's Russian sector, and take Anna with them as they leave. Martins then visits Dr. Winkel (Erich Ponto), a friend of Kurtz and Popescu and Lime's personal physician, who was present when Lime was killed. Martins questions the evasive Winkel about the circumstances surrounding Lime's death, and Winkel reassures Martins that there were only two men present at Lime's accident. Martins is not convinced.
The next day, the porter offers to give Martins more information about the death, but the man has been murdered just as Martins arrives to talk to him. Escaping from the hostile and suspicious crowd outside the porter's house, Martins is suddenly whisked away on a wild, careening ride — only to arrive at the book club meeting, where he is unable to collect his thoughts and makes a poor speech. His sole coherent response is to an inquiry from Popescu, who asks about Martins's next book; Martins pointedly replies that his upcoming novel is called The Third Man and will be inspired by actual facts. Popescu menacingly suggests Martins stick to fiction, and when he spies two suspicious-looking men advancing from the back of the hall, the writer flees.
The policeman Calloway advises Martins to leave Vienna and, when Martins refuses and demands an investigation into Lime's suspicious death, finally reveals the truth about Lime's racket. Calloway shows him a dossier and photographs proving that Lime stole penicillin — at the time a new and scarce life-saving antibiotic — from military hospitals and sold it on the black market. To maximise his profits, Lime diluted the penicillin, with devastating effects on his many victims. Martins, convinced, agrees to leave Vienna.
As he departs the police station, a Russian officer comes in and asks Calloway for Anna's forged passport in order to take her back to the Russian sector. Martins heads back to Anna's apartment to say goodbye and discovers that she too had been told by Calloway about Lime's activities. He also realises that he has feelings towards her. Leaving her apartment, Martins discerns a man watching from a dark doorway across the darkened square. A lighted window briefly illuminates the man's face, revealing him to be Harry Lime, alive and well. Unable to catch up with Lime when he flees, Martins summons Calloway, who determines that Lime has escaped into the sewer system through a kiosk. Calloway realises that Lime has been using the sewer tunnels to move about the city. Finally convinced that the wanted man is indeed alive, the British police exhume Lime's coffin and find another man, Joseph Harbin, an orderly in a military hospital, buried in his place.
The next day, Martins meets with Harry Lime in the Russian sector in the east of the city. They talk while riding in Vienna's famous Ferris wheel, the Wiener Riesenrad in Prater Park. Lime is dismissive about any effects of his activities. He offers to bring Martins in on his racket and implies that Martins will be disposed of if he causes further problems, but Martins rejects the suggestion and hints that he will not be easy to dispose of. Lime compares the people moving on the ground far below to dots, asks whether Martins would really feel pity if "one of those dots stopped moving forever" and whether he would really decline money — or would he calculate how many dots he could afford to spare.
Calloway asks Martins to help capture Lime. Martins agrees, negotiating Anna's safe conduct out of Vienna in return; but when she realises her freedom was the payoff in the deal to ensnare Lime, she leaves the train, refusing to play a role in his capture. Martins reconsiders his involvement, but when Calloway takes him to a hospital and shows him children crippled by meningitis after receiving Lime's diluted penicillin, he agrees to assist in drawing Lime out for them.
When Lime arrives at the rendezvous cafe, Anna calls out a warning. Lime evades capture and reaches the sewers, but police reinforcements have arrived and begin a mass search of the underground tunnels. He is eventually cornered and fires at Sergeant Paine, killing him, then is shot himself by Major Calloway in return. Lime, badly and perhaps fatally injured, drags himself up a staircase to a grating, but is unable to push it open. Martins, using Paine's gun, climbs the steps and shoots his old friend. In the aftermath, Martins attends Lime's second funeral. He waits by the roadside to speak with Anna, but she walks past without looking at him.
参考资料：wikipedia
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