The Third Man (1949)
Out of work pulp fiction novelist, Holly Martins (Joseph Cotton), arrives in post war Vienna to accept a job offer from ex-school friend, Harry Lime (Orson Welles), only to hear of Lime's death from a peculiar traffic accident.
Holly finds that Lime has faked his death and is involved in black market penicillin (diluted) that results in many deaths. He finds Lime and confronts him.
Based on Graham Greene's novel, the movie is faithful to it, but it was Orson Welles who added THIS little insight ...