American writer Holly Martins arrives in war-torn Vienna looking for his old friend Harry Lime. He is shocked to be told Lime has been killed in a car accident, and disbelieving when told Lime was a racketeer better off dead. He decides to investigate when he hears contradictory accounts of the accident, which includes reports of a mysterious "third man" who witnessed the accident. When a hotel porter he has spoken to is found dead, Martins has to flee an angry crowd and gets proof from a military policeman, Calloway, that Lime sold deadly diluted penicillin on the balck market. When he sees Lime lurking in the shadows he gives chase, but loses him. Through Lime's underground contacts, Martins finally meets with Lime, who tells him his amoral philosophy (as well as expounding on the difference between 1000 years of Italian and Swiss history). When Lime's girlfriend, Anna, a Czech refugee holding a false passport who still remains faithful to her lover's memory, is arrested, Martins suggests a deal with Calloway: Anna's freedom for capturing Lime. Anna furiously rejects the offer. After Calloway shows Martins some photographs of some of Lime's victims. Martins arranges another meeting with Lime, which culminates in one of the great set-pieces of cinema, a chase through the underground sewers.