At a Kolkata prison, Kathiresan "Kathir" (Vijay), a petty criminal, helps the police capture an escaping prisoner Vivek Banerjee (Tota Roy Chowdhury), but he himself escapes afterwards. He goes to Chennai and decides to escape to Bangkok with his friend and fellow-criminal Dhanu (Sathish). However, he drops this plan after meeting and falling in love with Ankitha (Samantha Ruth Prabhu) at the airport, who dupes him into believing that she wants to marry him after realising that he is having a crush on her. Later, Kathir and Dhanu notice a man who resembles Kathir named Jeevanandam "Jeeva" (Vijay), being shot by a group of thugs. They admit him to hospital, following which Kathir decides to impersonate the injured Jeeva to escape from the police. Under the name of his doppelgänger, Kathir and Dhanu enter an old-age home run by Jeeva with the aim of collecting INR25 Lakhs (which is meant for the inmates of the old-age home) for their Bangkok trip, until he learns about his doppelgänger's mission. Jeeva is a communist ideologue and a post-graduate in hydrology from the arid village of Thanoothu in Tirunelveli district who had discovered groundwater under some lands of the village which could be used as an irrigation source not only for the village, but for the entire Tiruneveli district and the neighbouring Thoothukudi district as well. But, an MNC owned by Chirag (Neil Nitin Mukesh) cheated the villagers into giving their lands for the construction of a factory. Jeeva took the issue to court, whose verdict is to be decided within a couple of days. On learning of the plight of Jeeva and the villagers, Kathir decides to fight for their cause while still under the name of Jeeva.
Meanwhile, the real Jeeva gains consciousness and finds himself in the Kolkata prison where Kathir was locked up. With the help of Vivek, who has heard his story and has plans to kill Kathir in revenge for getting him caught and thrown back into prison, he soon escapes along with Vivek's henchmen.
Not knowing that Jeeva and Vivek's henchmen are headed for Chennai, Kathir makes efforts to convince the media to bring the plight of the villagers to national consciousness, but the media is not interested as they feel that it is not a sensational news. A few days later, at the Madras High Court, the judge declares the verdict in favour of Jeeva and the villagers, but adds that Chirag has claimed that certain villagers who are working abroad have shown their support for the factory. If they cannot prove that their support was faked by Chirag within the next 5 days, the verdict will then go in favour of Chirag and the villagers will lose their lands. Since the villagers, who have denied supporting the factory, are abroad and cannot come to Chennai within 5 days to rebut Chirag's claims as their passports are held by their contractors, and the verdict is to be decided within 5 days, Kathir decides to take drastic measures to sensationalise the issue. He, Ankitha, Dhanu and the inmates of the old-age home block water supply to Chennai by sitting on the pipelines which carry water to Chennai from 5 lakes. With the plight of the Chennaites due to no water supply having gained national attention, Kathir comes out of the pipeline after a few days and highlights the villagers' plight in an emotionally-charged speech to the media, which is telecast nationwide and moves many people. By now, Kathir has discovered that Jeeva has escaped and is searching for him.
Meanwhile, Jeeva and Vivek's henchmen reach Chennai, but Jeeva is soon kidnapped by Chirag's henchmen. While in Chirag's custody, he sees Kathir's speech on television and is moved by the efforts made by his doppelgänger to help the villagers. On the night before the verdict, Kathir's bluff is exposed, but Kathir assures the old men that he is genuinely interested in their cause, that Jeeva is alive and that he will rescue Jeeva from Chirag and hand him back over to the villagers with the assurance of a positive verdict. Kathir goes to Chirag's office, where Jeeva is being held. He rescues Jeeva and fights Chirag and his henchmen, killing Chirag in the process.
The next day, the verdict is declared in favour of Jeeva and the villagers. Kathir, despite his efforts to thwart Chirag and the MNC, does not take part in the ensuing celebrations over the verdict and instead surrenders to the police and returns to the Kolkata prison, but not before promising Ankitha, who has now fallen in love with him, that he will marry her once he is released.