Onnaman movie is all about Ravishanker (Mohanlal) who is a street-urchin comes under the patronage of Salim Bhai (N. F. Varghese), a self-styled messiah of the masses and a don too. When Ravishankar grows up, he becomes an adviser to Salim Bhai. With the help of his gang of young friends, Ravishankar soon emerges as a leader of the poor and the oppressed. Naturally, the 'bad' guys led by Gulab Chand Sha, resent him and do everything possible to eliminate Ravishankar.