Director: A. R. Murugadoss
Cast: Vijay as Kathiresan Kaththi and Jeevanadam Jeeva, Samantha Ruth Prabhu as Ankitha, Neil Nitin Mukesh as Chirag, Tota Roy Chowdhury as Vivek Banerjee, Sathish as Ravi, Sudip Mukherjee as Kolkata police inspector, and Rama as Jeeva's mother.
In Kolkata, prisoner Kathiresan aka Kathir, a petty criminal, helps the police capture an escaping prisoner Vivek Banerjee, but he himself escapes afterward. He goes to Chennai and decides to escape to Bangkok with his friend and fellow-criminal Ravi. However, he drops this plan after meeting and falling in love with Ankitha at the airport, who dupes him into believing that she wants to marry him after realizing that he is having a crush on her.
Later, Kathiresan and Ravi notice a doppelganger of Kathiresan named Jeevanandam aka Jeeva, 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 doppelganger, Kathiresan and Ravi enter an old-age home run by Jeevanandam with the aim of collecting an amount of ₹25 lakhs which is meant for the inmates of the old-age home for their Bangkok trip.
But he learns about his Jeevanadam'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 neighboring Thoothukudi district as well.
But, an MNC owned by Chirag 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. How he solves all this and get water for the people, form the rest of the story.
Kaththi received a unanimous positive response. Critics highlighted Vijay's understated performance and the screenplay by Murugadoss. The movie is recorded at the box office as a hit. It is remade into Telugu as Khaidi No. 150 which also became a successful film.