On the night of 11 October 1943, the Japanese bomb the Indian city of Madras (now Chennai). The next morning in Triplicane, Rajan, a radio engineer and communications researcher, is found murdered with his own hand gun. His neighbour Chinnaiah Pillai hears the gunshot and makes a complaint to the police. Purushothaman Naidu, a local police inspector, arrives at Rajan's house and starts investigating the murder. In the meantime, Crime Investigation Department (C.I.D.) officer Sivanandam joins Naidu to help the investigation. Naidu suggests that the killer could be a thief who must have killed Rajan for the money found at the crime scene. However, Sivanandam is unconvinced with Naidu's idea because the sum of money present matches the withdrawal entry in the bank passbook found in the same room. Rajan was about leave Madras in anticipation of the bombings.
The two policemen question five people in and around Rajan's house, most of whom are family members or friends of Rajan. The first person to be questioned is Rajan's wife Usha, who is unable to speak during the inquiries. Sivanandam and Naidu feel embarrassed and are reluctant to question her further and they begin interrogating their next suspect, Chinnaiah Pillai, who reported the murder. Pillai proposes that the killer is probably Pattabi, Rajan's younger brother, and recalls a confrontation between Pattabi and Rajan. Pattabi asked for his share of the family property to be apportioned and given to him. Rajan refused to give Pattabi his share, feeling that he and his wife would squander it. Pillai concludes that this may have prompted Pattabi to kill Rajan.
Sivanandam and Naidu decide to interrogate Pattabi, who feels remorse for Rajan's death and states that he did not treat his brother well and failed to understand his good intentions. He recounts an incident in which Pattabi's wife Hema had fought with Rajan for not apportioning the property. Pattabi states that Hema could have killed Rajan for money as she loses sanity when overpowered by anger. Sivanandam briefly leaves Naidu to interrogate Hema. She is initially impudent and refuses to give a statement about the crime, but she later yields when threatened that her husband will be arrested. She reveals Rajan's extramarital affair with a dancer named Ambujam, who is pregnant with Rajan's child. As Rajan treated the news with a reckless attitude, Hema proposes that Ambujam could have killed Rajan. When questioned, Ambujam accuses Pillai of the murder, saying that he was her foster father who wanted her to stay away from Rajan, after the three met during a picnic. As their relationship continued, Pillai became infuriated and wanted to end the affair.
Sivanandam inquires Usha, who tells him how she and Rajan fell in love. Sivanandam tricks Usha using a leaky fountain pen to collect her fingerprints. That evening, Sivanandam meets all the suspects along with Naidu at Rajan's house and carries out an exercise in which the suspects—including Usha—must shoot Sivanandam as though he is Rajan using revolvers loaded with dummy bullets. All the suspects shoot, but Usha bursts into tears and fails to shoot. Sivanandam then orders an apparent arrest of Pattabi and Hema. Unable to bear the torture, Usha reveals the truth. The story goes into a flashback. Rajan is a radio engineer who wants to sell radios to the poor at an affordable price. Unable to get any support from the government, he goes to Japan where his work is appreciated. He becomes a spy, selling India's military secrets to the Japanese. Usha learns about this and tries to reform him. But Rajan does not mind betraying India. Usha cannot stop Rajan and tries to shoot him. She changes her decision but pulls the trigger accidentally, killing Rajan. After revealing the truth, Usha commits suicide.
