Sivakasi alias Muthappa (Vijay) is a do-gooder in Chennai who works as a welder there. He enjoys life with his friends and forgets the past he left behind. He falls in love with rich girl Hema (Asin), and her family accepts him as an honest, good man at heart. One day, he lashes out at his lover’s brothers and Hema yells at him. Sivakasi gives her one slap and tells her that he was not the way he currently is. He tells her about his past.
Muthappa was a good boy who lived in a village named Nattarasan Kottai and followed his parents’ orders. His elder brother Udayappa (Prakash Raj), who was a crook even as a child, tells him to get some fireworks for the village Deepavali festival. Muthappa obeys, and Udayappa lights the firework. A woman stands on top of the bursting firecracker and begins to burn. She begs for life and is carted to the hospital. Their father (Rajesh) interrogates Sivakasi. Udayappa tells Muthappa to not tell that it was him. Muthappa is left in a long period of silence. Udayappa screams to the village that it was Muthappa. Their father thrashes Muthappa. Muthappa’s sister Vairam (Lakshana) and mother (Geetha) cry for him to stop, but their father pays no attention. Udayappa smirks and laughs throughout the thrashing. Muthappa is disowned and his father tells him never to show his face to him again. Muthappa secretly obtains several objects during the beating, his mother’s bangles, the belt used by his father to thrash him, and his brother’s glasses. Ashamed and no longer knowing who he is, Muthappa leaves for Chennai, changes his name, and survives, with the sympathetic support of Venky (M. S. Bhaskar) and Mamoi (Chitti Babu), who, together with Sivakasi, help him forget his past and start a welding business. Sivakasi soon makes a living as a successful welder and becomes the owner of the largest welding shop in the city, respected by the local people and a local hero. As a result, he slowly lets his horrific past drift away from him.
Sivakasi is criticized by Hema for his actions, and she leaves him saying his family’s agreement will seal their marriage, and she will not marry anyone else but him until his family approves. Sivakasi finds the old possessions and realizes he has to return to make things right. Sivakasi leaves for his village and realizes how much has changed over the years. His father is now dead. His brother, now a MLA, treats his mother and sister very badly. After finding out the pitiable condition of his mother and sister, he decides to teach a lesson to Udayappa. He does not reveal his true identity to them. Sivakasi reconnects with his old friends and learns about how his father died. Udayappa molested a young girl and was beaten by his father. When his father questions how he got the bad habits he has, Udayappa retorts by asking his father to ask his wife who the offspring is. Sivakasi’s father dies of an instant heart attack. Sivakasi attacks Udayappa’s mansion by beating all his henchmen, and later on beating Udayappa with his father’s belt, and tells a false story that his wife had died because of him, then the police arrive at the mansion. Udayappa allows Sivakasi to escape arrest because Sivakasi blackmails Udayappa by showing him a letter of his wife, a suicide note. Sivakasi demands money in exchange of the letter, which it is later revealed that it was just a blank paper. Sivakasi earns the money, and turns in an empty piece of paper to Udayappa, who realizes he has been cheated. Sivakasi uses the money to fund his sister’s campaign. He is thrown by an enraged Udayappa.
Sivakasi returns to the village the next day and continues to support his sister in the election by exposing the misdeeds of Udayappa and blackmailing him at every corner. In the election, his sister wins by a huge margin, Udayappa losing his deposit. At his sister’s victory celebration, Sivakasi reveals that he is Muthappa. Udayappa angrily orders his men to beat the others surrounding Sivakasi and to take away his mother and sister. Udayappa then pours petrol all over Sivakasi and nearly burns him alive. Udayappa’s mother-in-law (Telangana Shakuntala) comes into the scene and catches the torch intended for Sivakasi. She and her men attack Udayappa. Sivakasi, angry but shocked, stands still until instructed by his mother that even though Udayappa had done many sins, they are still a family and should defend one another. Sivakasi goes to the rescue and saves Udayappa, while killing all the goons except for the mother-in-law. Udayappa begs for Sivakasi’s forgiveness as he was very mean to his brother, and they both embrace and reconcile. Udayappa begs forgiveness from his mother and she forgives him. Sivakasi then reveals his marriage plans to his family, and then Udayappa, his mother, his sister, and the entire village attend his and Hema’s wedding. Sivakasi becomes a celebrated hero in the village.