Holly Martins, played by Joseph Cotten, arrives in Vienna to visit his old friend Harry Lime, a black market dealer. However, he soon learns that Lime has been killed in a car accident and attends his funeral. There, Martins meets several of Lime's acquaintances, who tell him different stories about Lime's death. One of them mentions seeing a "third man" at the scene of the accident.
Martins becomes obsessed with finding out the truth about Lime's death and starts investigating on his own. However, he faces opposition from Major Calloway, played by Trevor Howard, a British military police officer who informs him that Lime was a criminal and a murderer. Calloway warns Martins to leave Vienna, but he refuses to give up his investigation.
As Martins continues his search for answers, he meets Anna Schmidt, played by Alida Valli, a stage actress and Lime's former lover who is devastated by his death. Martins falls in love with Anna, but she is torn between her loyalty to Lime and her feeling for Martins.
Martins eventually uncovers the shocking truth about Lime's death, which involves a complex web of conspiracy and betrayal. The film's famous climax takes place in the city's iconic sewer system, where Martins confronts the "third man" in a tense and dramatic scene.
The Third Man is considered a classic of film noir, with its atmospheric cinematography, evocative setting and memorable performances. The film's iconic zither music score, composed by Anton Karas, became an instant sensation and remains one of the most recognizable and beloved film themes of all time.