Annan Thambi is the story of twin brothers, Appu and Achu. Their father Ravunni, a ballet artist, separates them due to sibling rivalry, and Appu is sent to live with relatives in Pollachi. He grows up to be a lovable thug who falls for Thenmozhi, while Achu marries Lakshmi. But soon major misunderstandings created by their enemies crop up and the brothers are baying for each other's blood. Just before the climax, they come to the realization that others were behind their enmity, so they patch up their differences and take on their foes.