It is basically an investigative film as CBI officer Kasi Viswanathan (Vijayakanth) is entrusted to investigate the mysterious disappearance of Judge Sadhasivam (Nassar). The Kasi Viswanathan team comprises junior officer DCP Kesavan Nair IPS (Anandaraj) and a head constable Kandhasamy (Pandiarajan).

Soon Kasi finds out that a state minister Ilakkiyan (Prakash Raj) is behind all crimes in the city, and he is assisted by three senior cops. Suddenly, one by one, the bad cops are killed, with needle of suspicion resting on Kasi, as a lookalike is behind the murders. It is revealed that it is a revenge killing by Perarasu Pandiyan (Vijayakanth), Kasi's twin brother, and that Kasi's birth name is Ilavarasu Pandiyan.

We are told in a flashback by the family retainer Maarimuthu (Chandrasekhar) that Kasi was the long-lost twin brother of Perarasu Ilavarasu Pandiyan, and their father, the local chieftain Chakkaravarthi Pandiyan (Sarath Babu), was at one time the kingmaker of Panchalankurichi. After making Sivapprakasam (Mansoor Ali Khan) and Ilakkiyan MLAs, he falls out with them and they murder him. Perarasu, who has seen this murder, now wants to take revenge.

In the end, Perarasu fits a bomb on a chair in a meeting on which Ilakkiyan sits. Kasi, desperate to save Ilakkiyan, goes into the room and prefers to die with Ilakkiyan. Perarasu feeling proud for his brother preferring to give up his life for his duty, saves both of them, but is shot by Ilakkiyan. Kasi then shoots Ilakkiyan, and Perarasu takes the blame before dying. The film ends with Kasi marrying the heroine and returning to become the next chieftain of his native village.