Ilija Čvorović, a former Stalinist who spent several years in a prison on Goli otok, is contacted by the police to routinely answer questions about his sub-tenant, Petar Markov Jakovljević, a businessman, who spent twenty years living in Paris, and now has returned to Belgrade to open a tailor shop. After only several minutes, Ilija is free to go, however, he starts to suspect that his sub-tenant, Petar, might be a spy. As time passes, Ilija becomes convinced that Petar, a modern man from a capitalist country, represents a great threat to national security and the socialist system, and begins spying on Petar. Ilija's wife Danica is more concerned with the future of their daughter Sonja, who, although holding a degree in dentistry, is unable to find a job. After a bout of spying, Ilija phones inspector Dražić, claiming that Petar was meeting with "suspicious people", but Dražić does not take him seriously. Ilija decides to take matters into his own hands. He begins his own surveillance operation against the innocent man and his friends. Eventually, he bars his house, buys a guard dog, arms himself with munition, and even gets help from his brother Đura, both of them becoming convinced that Petar is a foreign agent. One evening, Ilija is accidentally hit by a car, which he sees as an attempt of assassination. Soon, even Danica starts to believe Ilija, but Sonja believes that her father is suffering from paranoia. Đuro manages to capture several of Petar's friends, holding them in his basement, beating them up and making them "reveal their terrorist plans". Petar comes to Ilija's house, where he finds Danica. Petar says that he wanted to say goodbye, as he is traveling to New York, and asks Danica why Ilija and his brother are following him, thus revealing that he was aware of their "surveillance operation". Ilija and Đura crash into the house, sending Danica away, tying Petar to a chair, beating him and forcing him to "confess". Petar keeps claiming that he is not a spy, but the brothers do not believe him. Đura leaves the house for a while, to bring one of Petar's friends who "admitted everything", and Ilija continues to interrogate Petar. However, Ilija gets too excited and has a heart attack. Petar manages to get to the phone and call the ambulance, and then, still handcuffed to the chair, he leaves the house to try to catch his plane. Ilija, while in severe pain, phones Đura's house and tells his wife to tell him to "block the airport". He then crawls out of the house, and starts crawling after Petar, with his dog following him.
