The story begins with a young couple, Balu and Ishwarya, being chased by some goons. Ishwarya is the daughter of Rajadorai (Charan Raj) and had eloped and run away with her boyfriend Balu against the wishes of Rajadorai. Rajadorai sends his goons to track down the couple. When the goons finally track down the couple in a forest and start beating up Balu, a man (Rajinikanth) comes to the couple's rescue. When Rajadorai and his brother Ramadorai (Nizhalgal Ravi) find out about this man, they rush over to his hideout in the forest, only to escape in fear when they see him. The couple is scared and puzzled as to how the man could induce so much fear in the brothers. The man's wife Parvathi (Gouthami) then tells the couple the story of the man, whose name is Dharmadorai.

Dharmadorai was a farmer in a village named Valliyur. He was the elder brother of Rajadorai and Ramadorai. He had showered so much love and affection on his younger brothers and even borrowed ? 30,000 for their "education", much against his father's (Madhu) wishes, who fully knew that his younger sons were crooked and evil. He even got Rajadorai married to Vaibhavi (Vaishnavi), a girl whom Rajadorai had raped, to save him from a jail term. The brothers however, showed no sort of affection for Dharmadorai and used his innocence for their sinister desires. Rajadorai convinced Dharmadorai to borrow ? 6 lakhs so that he could "start a business". Using this money, Rajadorai and Ramadorai moved to Madras (now Chennai) and went into smuggling. They later double-crossed their smuggler boss Jose by keeping all the items meant to be smuggled and also lodging a police complaint against his son Ajay (Ajay Rathnam) for their own benefit and soon became rich due to their illegal activities.

One day, Dharmadorai went to Madras when he heard that Ramadorai was going to get engaged to a rich girl and leave with her for the United States to "pursue higher studies". The brothers did not receive him warmly and humiliated him that evening at a farewell party organised for Ramadorai. Later that night, Ajay, who had been released from jail, entered their house to kill Ramadorai in retaliation for having double-crossed Jose and his gang. Ramadorai killed Ajay and convinced Dharmadorai to take the blame for the murder, who was sentenced to seven years' imprisonment as a result. The brothers then got Jose and his henchmen also arrested. While Dharmadorai was in jail, Parvathi gave birth to a baby boy.

A few years later, Rajadorai and Ramadorai decided to sell off their father's house and property to "repay the ? 6 lakhs Dharmadorai had borrowed". When their father opposed their move, the brothers killed him and drove Parvathi and her son out of the house. With nowhere else to go, Parvathi and her son moved to Madras. The small boy was also eventually killed by Ramadorai when he recognised him as the murderer of his grandfather. With no home, her husband imprisoned and her son dead, a grieving Parvathi took up the job of a housemaid in Madras. Soon, Dharmadorai completed his jail sentence. When he found out about Parvathi's plight and how his brothers had betrayed him, he rushed over to the brothers' house and thrashed them mercilessly, only stopping when their wives intervene. Following this, he disowned them and moved to the forest with Parvathi.

In the present day, Jose and his henchmen get released from jail and kidnap Rajadorai and Ramadorai in retaliation for betraying them. On Parvathi's advice, a reluctant Dharmadorai goes to rescue his brothers. He rescues them, following which they seek his forgiveness, which he accepts.

• Rajinikanth as Dharma Durai
• Madhu as Rajinikanth's father
• Gouthami as Parvathi Dharmadora
• Nizhalgal Ravi as Ramadorai
• Charan Raj as Rajadorai
• Vaishnavi as Vaibhavi
• Ajay Rathnam as Ajay
• Venniradai Moorthy
• Mohan Raj
• Senthil
• Dilip (Tamil actor)
• Bailvan Ranganathan

Dharma Durai was initially started under the title "Kaalam Maaripochu" and with different storyline. They started the shooting with Rajini, Manjula, Ramesh Aravind and Senthil etc. But later, they dropped the movie and started to remake Kannada film Deva as Dharma Durai. When Dharmadorai wasn't able to release on time due to certain circumstances, based on Rajini's personal request, Rajasekhar took it as challenge and worked continuously which include shooting, dubbing and re-recording. This was a real achievement and commitment by Rajasekhar which saw the film release on time.

Dharmadorai became a big blockbuster and a clear winner among 1991 Pongal-released films. On the 100th day of Dharmadorai, director Rajasekhar died.^[5]

The soundtrack was composed by Ilaiyaraaja while the lyrics for all the songs were written by Panchu Arunachalam. Gangai Amaran wrote only one song, Aanenna pennena.^[6] Ilayaraja reused "Maasimaasam" as "Enno Rathri" in Telugu film Dharmakshetram (1992).^[7] The song was later remixed by Srikanth Deva in Pandi (2008).^[8]