After World War II, American writer Martin (Joseph Cotten) was invited by his friend Harry (Orson Welles) to work in Vienna. Unexpectedly, as soon as he arrived at his destination, he heard the news that Harry had died in a car accident. At Harry's funeral, Martin met the local officer Carlovi, who asked him to leave Vienna immediately. Martin ignored him and decided to stay and investigate the truth of his friend's death. Martin found Harry's girlfriend Miss Scolari (Alida Valli) to investigate the truth of the incident together. As the investigation deepened, they found that the inside story became more and more confusing. First, their landlord was killed, and then Martin found that he was being followed, and he found out that there was a third person present when Harry died. So, the key to everything lies in finding this mysterious third person.