Having believed a police officer's life to be luxurious, with much income in the form of bribes, Karnan, an orphan, grows up to become a corrupt and ruthless Sub-Inspector. He forms an immediate friendship with the local don ECR Kaalirajan by releasing his four brothers Kumar, Prakash, Vinay, and Sabari from lock-up, who were all arrested for smuggling. Karnan's attitude does not go well with his subordinate Abdul Kader, a sincere police constable. In vain as it is, he tries to oppose Karnan's deeds.

Meanwhile, Karnan meets and falls in love with Sindhu, who gets kidnapped by Kaalirajan's men on her birthday but is rescued in time by Karnan. Kaalirajan then scolds his henchmen for kidnapping the wrong person and apologizes to both of them. As her birthday gift, Sindhu requests Karnan to save the girl who was supposed to be kidnapped and killed by Kaalirajan. A reluctant Karnan fights Kaalirajan's goons and saves the girl, Sandhya. Karnan somehow reconciles with Kaalirajan later and learns that Sandhya has certain evidence against Kaalirajan's brothers. Karnan meets her and learns that her sister Bhavani was kidnapped, raped, and brutally assaulted to death by Kaalirajan's four brothers for two weeks, and this act was recorded by them. The footage was stored on a camera, which is with Sandhya now. Karnan takes the camera and arranges to send both Sandhya and her mother to the States.

Before their departure, Sandhya's conversation with Karnan leaves him shattered. Realizing that he was indeed responsible for the rape since he allowed the brothers to escape on the day they kidnapped Bhavani, Karnan is devastated and soon realizes that he has reformed after he fights off Kaalirajan's goons sent to retrieve the camera at the police station, with Kader notifying the change in him. He soon manages to track the brothers and beat them all up before arresting them. After Bhavani's corpse is found and the autopsy is performed, Karnan produces the camera as the evidence, which turns out to be an empty one. Though no further evidence exists, Karnan asks for a gap of one day to provide necessary evidence so that Kaalirajan's brothers do not escape as exonerated. Later, Kaalirajan's men attack a drunk Karnan when he is alone. After a brutal fight, they bury him alive, but he manages to escape.

Karnan reaches the court the next day, and to everyone's shock, he adds that he is one among not the four, but "five" men who sexually assaulted Bhavani. He then convinces the judge to sentence all five of them to capital punishment in order to ensure that the other four do not escape punishment. This statement shakes the entire course of the case, and Karnan is publicly resented. He is allowed by the commissioner to watch as all four of the brothers are hanged, after which he, too, is hanged. Merely hours later, Sandhya and her mother arrive in Chennai, publicly revealing Karnan's innocence and the sacrifice he made to ensure punishment to all of the accused criminals. The revelation shocks the nation, and the public who misunderstood him eventually mourns his death as his corpse is brought and walked from the gallows. The film ends with Karnan's final words; "All men will now fear an immediate death sentence, if they ever molest a girl."