In Madurai, veteran rooster trainer Periyasamy (60) aka "Pettaikaaran" (V. I. S. Jayapalan) and Madurai Central Police Inspector Rathnaswamy (40) (Naren) are tough competitors in cockfights, and it is often Pettai who wins because nobody knows his way of maintaining the quality of the birds. Rathnasamy and Pettaikaran were once proteegees of grand veteran Doraisamy, who has retired from active competition, but is the overlord of rooster fighting in South Tamilnadu.
Karuppu (Dhanush) (26) and Durai (30) (Kishore) are the favorites in Pettai's team. Karuppu is very talented in breeding and training roosters, while Durai is very rich, owns three bars in the town of Thiruparangundram, and is also talented in cock training. Rathnaswamy keeps insisting on having one last fight to win and satisfy his old mother’s wish, but Pettai declines to have any more fights with him because he feels Rathnaswamy has lost faith in his roosters and will be using nefarious methods to win. Rathnaswamy keeps insisting and tries to make him accept the challenge through cajoling, threatening, bribing and other vile acts.
Karuppu falls in love with an Anglo-Indian girl, Irene (Taapsee), who dislikes him. He does not recognize this initially and keeps following her. One day, the residents of Irene's colony confront him and ask Irene to tell who she is in love with – Karuppu or Dinesh (Dinesh Ravi), another man in the locality. She points her finger at Karuppu and he goes into rapture. She explains later that she had to lie in order to avoid Dinesh who has been bothering her for a long time. Later, Irene too falls in love with Karuppu.
Meanwhile, Rathinasamy tries to cajole Ayubh, pettaikaran's veterinarian for roosters, to get pettaikaran come for a competition against himself, in exchange for a heavy amount. Ayubh refuses and insults Rathnasamy. The same night, Ayubh is killed in a hit and run accident. Durai initially suspects Rathnaswamy, and all of Pettai's gang wants revenge. But, Pettai decides to conduct a state tournament in Ayubh's name and provide his family with funds, so that Ayubh's poor daughters can be married. He asks permission from Rathnasamy, as the police have to permit it, but Rathnaswamy refuses, manhandles and berates Pettai as a thief. In a rage Pettai, suspecting Rathnaswamy of the murder, agrees to the one-on-one rooster fight Rathnaswamy has been asking for so long. He bets that, he will field his roosters against every rooster Rathnaswamy's brings to the field in the following tournament and even if one of Pettai's roosters loose against Rathnaswamy's, pettaikaran will tonsure his head and face, publicly apologies to him, and will quit rooster fights. But, if Rathnaswamy is not able to beat at least one of his roosters, the same conditions will be applied on him, at the end of the day. Finally getting his way, Rathnaswamy permits the tournament.
The grand state tournament is arranged by Pettai's team, getting heavy funds and an official permission. In the initial 11 fights, Rathnasamy's roosters are deafeated soundly by pettaikaran's. Rathnasamy brings high breed roosters from Bangalore, and enters them into the tournament. Seeing the quality of new roosters, Karuppu asks Pettai to let his rooster fight, but Pettai puts his rooster down and says he will choose the best competing rooster himself. Karuppu surreptitiously enters the contest as Pettai's team before Pettai can choose the rooster because he needs to repay Irene for the loan he took to prepare his rooster for the fight. Pettai does not believe in Karuppu and his rooster, announces that Karuppu will not represent Pettai, and the result of Karuppu's match will not be acknowledged by his team. The 'underdog' however emerges victorious in three consecutive rounds despite facing roosters spiked with steroids. Karuppu gets the best coach award of the tournament and his bet money of 3 lakh 10 thousand?10 lakh (equivalent to ?15 lakh or US$22,000 in 2017).
While initially happy that Karuppu won, Pettai is soon overcome with anger and jealousy. His ego is hurt by the fact that Karuppu earns both popularity and money, by refuting his judgement. Karuppu is not aware of the changes in Pettai's mind. Pettai is at first refusing to speak to anyone and then starts scheming. Karuppu gives to Pettai the money from the competition for safe keeping, in which he plans to use to start a business. But, it suddenly goes missing. Pettai also starts spinning stories about his associates and makes everyone suspicious of each other. He incites Karuppu and Durai into pitting their roosters in a fight. Meanwhile, Karuppu's mother dies from the shock of losing all the money. Meanwhile, Pettai poisons all of Durai's roosters making him suspect Karuppu, who is arrested for this. Pettai then lies to Irene that his wife was in an adulterous relationship with Karuppu. Irene's family also pressure her to move to Chennai which pushes her into attempting to commit suicide. Pettai meanwhile calls Karuppu to a location near a temple and informs Durai about his whereabouts. When Irene gets better, she calls Karuppu and informs him that Pettai is badmouthing him. So, Karuppu goes to find Pettai and confront him. Karuppu finds that Pettai was the one who stole his money, and a guilt-ridden Pettai kills himself. Karuppu does not wish to reveal the fact to the public that Pettai stole his money and schemed against him, so he flees with Irene to start a new life in another city leaving the money with his friend.