A young journalist named Bharathi, along with two of her friends, investigates a series of terrorist attacks which have taken place in Chennai and also illegal activities like human trafficking. When they are caught video-recording illicit activities happening in a secluded house one night by a group of thugs, they attempt to flee from them. While Bharathi's friends are killed, Bharathi manages to escape despite being stabbed by one of the thugs, but after the attack on Bharathi and her friends, the thugs' vehicle explodes due to a small mishap when a lit cigarette falls over petrol packets stored in the vehicle, killing all the thugs. Bharathi comes up with a new idea to eradicate society's evils by creating and propagating a fictional character called "Velayudham" who wants to clean up the city of terrorism and other illegal activities.

It shifts to a village called Pavunoor, where a milk vendor named Velayudham alias Velu is residing with his younger sister Kaveri, whom he loves dearly along with his cousin Vaidehi. One day, Velu, Kaveri, and Vaidehi leave for Chennai to collect money from a chit fund for Kaveri's marriage. At the Chennai Egmore railway station, Velu's handbag is stolen by Speedu, a petty thief. Velu pursues Speedu on a motorbike parked at the station and manages to get back his money, but moments after alighting from the bike, the bike explodes. The parked bike had a bomb which was meant to ensure a terrorist attack at the railway station, and thus, Velu inadvertently thwarted a deadly terrorist attack which would have killed several people. After a few more incidents where Velu unknowingly prevents more terrorist attacks, he comes to Bharathi's attention. At first she hates him due to various incidents But when she saw him unknowingly handing over the terrorists to the police, she falls in love with him without knowing that Vaidehi also loves him. When she learns that he unknowingly prevented the attacks, she convinces him to take up the role of Velayudham and provide hope to the people. But he refuses to do it and tells her to finish of the avatar of Velayudham. Soon he learns that his money is stolen and the company where he insured the money was a corrupt company. The people gets angry when they learn that the company is a corrupt company and their money stolen. One of them commits suicide by burning himself alive as he need money for his daughter's operation and if he didn't get the money his daughter would die. Velu tries to save him but it fails. When the villagers tells that Velayudham will come and help them he plans to take the role of Velayudham. He kills the person who stole his and the people's money and returns the money to the people. In the avatar of Velayudham, Velu starts to eradicate the evils that pertain within the society, such as corruption, prostitution, and terrorism, among others. He soon gains wide support and acceptance among the public and even the police.

A few months later, Velu returns to Pavunoor with Bharathi for Kaveri's marriage. Unknown to Velayudham, Musafir Ibrahim, who is the leader of a terrorist group followed him to his village. Ibrahim wants to kill him since he, in the guise of Velayudham had thwarted every terrorist attack that he had planned in Chennai. On the day of Kaveri's marriage, a bomb planted by Ibrahim at Velu's house explodes, killing Kaveri.

A few days after Kaveri's death, a grieving Velu learns from Bharathi that Ulaganathan, the corrupt Home Minister of Tamil Nadu, is indirectly responsible for the terrorist attacks and Kaveri's death. Ulaganathan allowed Ibrahim to carry out terrorist attacks in exchange for money. He even tries to gain popularity by claiming Velayudham as his idea, planning to reveal Ibrahim as Velayudham's real identity in a function at the Nehru Stadium, Chennai. Velu returns to Chennai, kills Ibrahim, exposes Ulaganathan's misdeeds. Ulaganathan is killed in a stampede and Velu returns to his village, promising that he will return as Velayudham again.