Thaimagan is a story of an investigative journalist Dheeran (Sarathkumar) who is intent on exposing corrupt minister Shanmugavadivelu (Mukesh Tiwari) and his stooge, the state director general of police (DGP) Alangaram (Seema Biswas).
The story revolves around a bottled water plant in a village sanctioned by the minister for a quid pro quo. Dheeran is against the project as it would deplete the ground water resources and warns the people and government about its dangers.
The minister sends his goons to silence the newspaper owner (Vijayakumar) permanently. But the Dheeran takes up the challenge, and continues his tirade against the minister's corrupt practices.
In an act of revenge, Alangaram plants a bomb in the car in which Dheeran is traveling. The bomb rips apart his co-passengers, but Dheeran lands on a goods train heading to Kerala. He is given medical care there, and two years later, he returns to Chennai to hunt down his enemies.