A trade union leader in hiding is found through his son Velu and killed by the police. Velu, who innocently believed the police and exposed his father, takes revenge by killing the policeman and flees to Bombay. He develops a friendship with another homeless boy Selva, who introduces him to slum life. Brought up by a kind-hearted Muslim fisherman along with his daughter Shakila (Thara), he grows up as Velu Naicker (Kamal Haasan) operating from a ghetto. When he kills a much-hated police officer in a fight for killing his foster father, he instantly becomes a hero in the eyes of the slum dwellers. Naicker soon becomes a Godfather of that area, engaged in smuggling, and at the same time supporting the downtrodden. When he visits a brothel, he comes across a beautiful and timid young girl Neela (Saranya), who gets impressed by her dedication to continue her studies despite being in that profession and marries her. When a builder buys the land on which the slum is situated, Naicker attacks his house and forces him to destroy the title deed, thus saving the slum from being demolished. This action makes him a still bigger hero in the eyes of the slum dwellers. As his popularity and clout increase, he starts eliminating rival gangs. Selva (Janakaraj) and lyer (Delhi Ganesh) are his close associates in these activities.
Velu Naicker becomes so powerful that even police officers approach him for help. He leads a happy life with his family: his wife Neela, son Surya and daughter Charumathi. Once, when nobody is able to bring a contraband parcel out of a ship, Velu goes with Selva and brings it ashore by hoodwinking the Customs officials. The Reddy brothers (Sudarshan and others), who were unable to bring out the same contraband, lose their top position in the dockyards to Velu due to this. Angered, they decide to kill him and his family. In their attack, his wife Neela gets killed. Velu retaliates by killing every member of the Reddy family. Worried about the safety of his children, he sends them to Chennai to be with his sister Shakila. Years pass and Velu grows to be the biggest mafia leader of the city with a huge number of followers. His children Surya ('Nizhalgal' Ravi) and Charumathi (Karthika) return to Bombay as adults. Despite Velu's reluctance, Surya follows his father's footsteps. Charu gets upset about this development and expresses her anguish.
One of Velu Naicker's associates turns approver. Surya hires a professional killer, who kills the approver in the court. In his excitement, this killer comes to meet Surya personally at a petrol depot to report his deed. When the police surround them, Surya burns the petrol depot to escape and destroys all the evidence but is killed in the process. Charu gets angry, blames her father for the death of her brother, and leaves him. More years pass by and Velu continues to dominate the area. A new Assistant Commission of Police (Nasser) takes charge of the area and starts taking severe action against Velu's activities. He arrests many and seizes their vehicles. When Velu goes to his residence to caution him, a surprise awaits him. His daughter is the ACP's wife and they have a son. Velu understands the situation and leaves. The ACP continues his punitive actions against Velu. However, he is unable to arrest Velu since the people in the area protect him and even set themselves on fire when the police approach his hideout.
Worried about his people dying, Velu calls his daughter and advises her to ask the ACP to come and arrest him after he completes the ceremony relating to the death anniversary of his wife the next day. By then, the ACP comes to know that his wife Charu is none other than the daughter of Velu. When he reaches home, Charu advises him to arrest her father the next day. The ACP arrests him but due to lack of evidence, he is acquitted by the court. When he comes out of the court, a mentally deranged youth Ajith (Tinnu Anand), son of a police officer whom Velu Naicker had killed years back, and whom he had taken under his care, shoots Velu dead.
The film belonged to the cinematographer, Sriram, and art director, Thottaa Tharani. For Tamil cinema, this was a new development. They both endowed the film with cinematic qualities that were rarely seen in an Indian film. Their attempts to evoke period flavour through automobiles and furniture pieces were new to Tamil filmic tradition. Sriram won the national award for cinematography and Tharani for art direction. It was selected as the Best Film of the Year by the National Awards Committee. The film won other honours also. Kamalahasan's screen portrayal of the ruthless don received much favourable notice and won him the country's best actor award for the year.
[from the book Pride of Tamil Cinema: 1931 to 2013, by G Dhananjayan, Blue Ocean Publishers, 2014]