Thangarathnam is a film, which is based in a village. The hero, a tempo driver carries vegetables from the nearby village to the Ottanchathram Gandhi market where the fundamental rule is that whoever gets the vegetables first, gets the best price. This leads to constant clash between the hero and another tempo driver. How the rivalry between the two affects the love life of the hero and how he comes out of the situation forms the crux.
Suriya is a spoiled brat who deceives girls in the name of romance. Kousalya is one of the victim who falls into his trap. One day, Suriya gets kidnapped and finds himself trapped in a wooden box akin to a coffin. Following Suriya's parents complaint, ACP Devendra Kumar (Rahman) takes up the case. What happens in course of investigation forms the rest of the story.