Attagasam movie begins with Jeeva (Ajith) who is a driving instructor leading a happy life with his mother Sujatha. She hides from him the truth that his father, Nizhalgal Ravi who was murdered by a dadha, Babu Anthony. The mother much against the boy’s wish packs off Ajith’s twin brother, Guru ( Ajith) who had been witness to the murderer from a close quarter, to a distant town. He goes to Tuticorin and grows to become a gangster there.
When the younger Jeeva visits Tuticorin, he comes across his elder brother. Seizing the opportunity, the elder brother kidnaps the younger one and decides to go as the driving instructor to Chennai. Initially, he plans to take revenge on his mother for packing him off from their house at a very young age. However, upon coming across Babu Anthony, he takes up a mission to kill him.
Meanwhile at Tuticorin, the younger brother sets things right for the elder one.
The movie ends on a happy note with the two brothers doing all that film heroes are supposed to do --- kill the evil ones, marry the lady love and make the mother happy.