Thalaimagan 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 hero takes up the challenge, and continues his tirade against the minister's corrupt practices.

In an act of revenge, Seema Biswas 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.

Pretty heroine Mekala (Nayantara), the hero's reporter colleague, comes to him like a balm.

A sting operation is also staged by Dheeran. Under the guise of a handicap, he joins the bottled water plant as a cleaner. He records the boardroom discussions between the minister and the foreigners where the former demands a bribe for sanctioning more plants.

He then passes on the information to various news channels, the state governor and the minister as well. Meanwhile, the minister locks the villagers inside the factory and threatens to gas them all.  Will the hero save the villagers and bring the minister to justice? Watch the film to find the answer to this age old mystery.

Uppumotai (scored by debutant music director Paul Jacob) and Nooru Nooru (by Srikanth Deva) are good songs. Stunt master Stun Siva does an interesting fight sequence at the start. However, Vadivelu's comedy does not induce laughter.

However, there are a couple of bloopers in the film. First, the hero is shown as the eldest son of Vijayakumar, the newspaper owner. Later, it turns out that Dheeran is an orphan. The other blooper in the film is how the heroine tracks down the villain's secret files.  On the whole, Thalaimagan is like reading a morning paper in the evening!