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 a rich girl Hema (Asin) and her family accepts him as an honest and 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 who 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. What happens after this incident and how muthappa becomes sivakasi to be watched.