In Kolkata, Kathiresan is a prisoner, who helps the police capture an escaping prisoner Vivek Banerjee but escapes afterwards.

Kathiresan goes to Chennai to escape to Bangkok with his friend Ravi. However, he drops this plan after meeting and falling in love with Ankitha, who dupes him into believing that she wants to marry him after realising that he is having a crush on her and gives him a false phone number. Later, Kathir and Ravi notice Jeevanandham, who resembles Kathir, being shot by a group of thugs. They admit him to a hospital, and Kathir decides to impersonate the injured Jeeva to escape from the police. Under Jeeva's name, Kathir and Ravi enter an old-age home run by Jeeva with the intention of stealing ₹25 lakh (US$31,000) from there for their Bangkok trip, until Kathir learns about Jeeva's mission.

Jeeva was a communist ideologue and 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. However, a greedy entrepreneur named Chirag cheated the villagers into giving their lands for the construction of a factory. Jeeva got arrested, and six villagers committed suicide to bring this news to the media forefront and get Jeeva released.

Moved by Jeeva's past, Kathir decides to fight for the villagers' cause while still posing as Jeeva. The people of the old home and Kathir go to the jury and try to convince him to go in favour of them, but Kathir starts stating rules and scares the main jury person (whom Chirag had bribed). Kathir 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 Kathir, but he defeats them. Meanwhile, Jeeva finds himself in the Kolkata prison. With the help of Vivek, who has heard his story and plans to kill Kathir, he escapes with Vivek's henchmen. Unaware that Jeeva is headed for Chennai with Vivek's henchmen, Kathir makes efforts to convince the media to bring the plight of the villagers to national consciousness, but the media is uninterested as they feel 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 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 go in Chirag's favour. Since the villagers who have denied supporting the factory are abroad and cannot come to Chennai so soon to rebut Chirag's claims as their passports are held by their contractors, Kathir decides on drastic measures to sensationalise the issue. He, Ankitha, Ravi, and the old-age home inmates block water supply to Chennai by sitting on the pipelines which carry water to Chennai from five lakes. With the plight of Chennai people due to no water supply having gained national attention, Kathir comes out of the pipeline after a few days and highlights the villagers' plight to the media. His speech is telecast nationwide.

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 kidnapped by Chirag's henchmen. While in Chirag's custody, he sees Kathir's speech on television and is moved by his efforts to help the villagers. On the night before the verdict, Kathir's bluff is exposed, but he promises the villagers that he will surely help them. He tells them that Jeeva is alive, that he will rescue him from Chirag and hand him back to the villagers with the assurance of a positive verdict. Kathir goes to Chirag's office, rescues Jeeva and kills Chirag. The next day, the verdict is declared in favour of Jeeva and the villagers. Despite the success, Kathir does not take part in the ensuing celebrations. Instead, he surrenders to the police and returns to the Kolkata prison after promising Ankitha, who has now fallen in love with him, that he will marry her once he is released.