On the night of 11 October 1943, during World War II,[2] Japanese forces bomb the Indian city of Madras (now Chennai). The next morning, Rajan, a radio engineer and communications researcher, is found shot dead by his own handgun in his house in Madras after his neighbour, Chinnaiya Pillai, having heard the gunshot, informs the police. Purushothaman Naidu, a local police inspector, arrives at Rajan's house and starts investigating the death.

Criminal Investigation Department (C.I.D.) Officer Sivanandam joins Naidu to help with the investigation. Naidu suggests that the killer could be a thief who killed Rajan for the money found at the crime scene. However, Sivanandam is unconvinced by Naidu's theory because the amount of money at the scene matches the withdrawal entry in the bank passbook found in the same room. Rajan was about to leave Madras in anticipation of the bombings.

The two policemen question five people in and around Rajan's house, most of whom are his family members or friends. The first to be questioned is Rajan's wife Usha, who is unable to speak because of her grief. Sivanandam and Naidu feel embarrassed and are reluctant to question her further. They begin interrogating Chinnaiya, who proposes that the killer is probably Pattabi, Rajan's younger brother, and recalls a confrontation between them: Pattabi asked for his share of the family property but Rajan refused his request, feeling that he and his wife would squander it. Chinnaiya concludes that this may have prompted Pattabi to kill Rajan.

When Sivanandam and Naidu interrogate Pattabi, who feels remorse for Rajan's death, he admits that he did not treat his brother well nor understand his good intentions. He recounts an incident in which his wife Hema fought with Rajan because he refused to apportion the property. Pattabi says Hema could have killed Rajan for the money as she loses her sanity when overpowered by anger.

Sivanandam leaves Naidu to interrogate Hema. She is initially impudent and refuses to give a statement about the crime, but she later agrees when Sivanandam threatens to arrest her husband. She reveals Rajan's extramarital affair with Ambujam, a dancer who is pregnant with his child. As Rajan treated the news of the pregnancy with a reckless attitude, Hema suggests that Ambujam could have killed him. When questioned, Ambujam accuses Chinnaiya of murdering Rajan, saying Chinnaiya was her foster father and wanted her to stay away from Rajan after the three met during a picnic. As their relationship continued, Chinnaiya became frustrated and wanted to end the affair.

Sivanandam interrogates Usha, who tells him how she and Rajan fell in love. Sivanandam tricks Usha into using a leaky fountain pen to collect her fingerprints. That evening, he and Naidu meet all the suspects at Rajan's house. Sivanandam carries out an exercise in which the suspects—including Usha—must pretend he is Rajan and shoot him using revolvers loaded with fake bullets. Chinnaiya, Pattabi and Hema shoot him as instructed, but Usha fails to do so and bursts into tears. Sivanandam then demands to have Pattabi and Hema arrested, until Usha reveals the truth.

Rajan was a radio engineer who wanted to sell radios to the poor at an affordable price. Unable to get any support from the government, he went to Japan where his work was appreciated. He then became a spy selling India's military secrets to Japan. Usha learnt about this and tried unsuccessfully to reform him. She could not stop him and attempted to shoot him. She changed her mind but pulled the trigger accidentally, killing him, and then cried at his boots later, holding them.

Sivanandam and Naidu ask Usha for the papers Rajan used to hatch the bombing plan. After she goes to Rajan's room to fetch them, Sivanandam tells Naidu that Usha's fingerprints which he collected and the ones on Rajan's boots matched, which led him to suspect her, and her failure to shoot during the exercise confirmed his suspicion. When Usha takes a long time to come back, the two hear a moment's silence followed by a gunshot, run into Rajan's room and Sivanandam screams, "Usha!".