''Annan Thambi'' is the story of twin brothers, Appu and Achu (both played by Mammootty). Appu is a talkative conman who always finds himself in trouble. While Achu, a mute, is nicer than his brother, he draws a dagger on Appu due to misunderstandings created by the villain Govindan (Siddique (actor)), who is masquerading as his friend. Their father Ravunni (Janardhanan (actor)), a ballet artist, separates them due to sibling rivalry, and Appu is sent to live with relatives in Pollachi.