Thirumagan story starts in a remote 'karisal kaadu' near Theni. Thangapandi (Suryah) is the only son of Malaisamy (Vijayakumar). He falls for the simplicity of a poor woman Aiyakka(Meera jasmine). Finally they plan to marry.
Thangapandi’s relative Rasaathi(Preethi verma) who covets for Thangapandi. In the meantime, marriage is on cards for Thangapandi, when his father promises his friend Sivalingam(Radha Ravi), to get his daring and rich daughter Lakshana, married off to Thangapandi. Seeing Thangapandi’s bravura act, even Maina(Mallavika) dreams hot to win his love.
Situations in the family turns bad when he told his love. Malaisamy takes his last breathe without fulfilling his friend’s promise. Nevertheless, Thangapandi and Aiyakka both will unite or not forms the rest of the story.