In Kolkata, prisoner 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 Ravi (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, Kathiresan and Ravi notice a doppelgänger of Kathiresan named Jeevanandam "Jeeva" (also Vijay), being shot by a group of thugs. They admit him to a hospital, following which Kathiresan decides to impersonate the injured Jeevanandam to escape from the police. Under the name of his doppelgänger, Kathiresan and Ravi enter an old-age home run by Jeevanandam with the aim of collecting ₹25 lakhs (which is meant for the inmates of the old-age home) for their Bangkok trip, until he learns about his doppelgänger's mission. Jeevanandam 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 Tirunelveli 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. Jeevanandam gets arrested by the police and six villagers commit suicide to bring this news to the media forefront and release Jeevanandham from prison. On learning of the plight of Jeevanandam and the villagers, Kathiresan decides to fight for their cause while still under the name of Jeevanandam. The people of the old home and Kathiresan go to the jury and try to convince him to go in favour of them, but Kathiresan starts stating rules and scares the main jury person (whom Chirag had bribed). Kathiresan then sends a person (disguised as a hairdresser) to Chirag and makes him lay a fingerprint on Chirag's neck. After that Chirag sends 50 men to kill Kathiresan but he soon defeats them using the coins the villagers gave when he was attending a ceremony for him.
Meanwhile, the real Jeevanandam gains consciousness and finds himself in the Kolkata prison where Kathiresan was locked up. With the help of Vivek, who has heard his story and has plans to kill Kathiresan in revenge for getting him caught and thrown back into prison, he soon escapes along with Vivek's henchmen.
Not knowing that Jeevanandam and Vivek's henchmen are headed for Chennai, Kathiresan 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 Jeevanandam 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 five 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 five days to rebut Chirag's claims as their passports are held by their contractors, and the verdict is to be decided within five days, Kathiresan decides to take drastic measures to sensationalise the issue. He, Ankitha, Ravi and the inmates of the old-age home block water supply to Chennai by sitting on the pipelines which carry water to Chennai from five lakes. With the plight of the Chennaites due to no water supply having gained national attention, Kathiresan comes out of the pipeline after a few days and highlights the villagers' plight in an emotionally charged speech to the media, which is tele