Mudhal Mariyadhai begins with friends and relatives of Malaichami (Sivaji Ganesan) gathered around him in a hut. The story is narrated in a flashback mode.
Malaichami's cousin Ponnatha (Vadivukkarasi) conceives a child out of wedlock and her partner is not known. To save his uncle from disgrace, who falls at Malaichami's feet and begs him to marry his daughter, Malaichami marries Ponnatha. Ponnatha looks down upon Malaichami and keeps repeating that he married her for her father's wealth. Though they live under the same roof, they don't share a man-wife relationship. He brings up Ponnatha's daughter Rasamma (Aruna), not letting her know that he is not her biological father and also gets her married. He also brings up his orphaned nephew Chellakannu (Deepan) who assists him at his farm. Years roll by. Malaichami, the village chief, is respected by one and all. Kuyil (Radha) a young lady, comes to his village along with her father looking for a livelihood. They build a hut on the river bank and Kuyil rows a coracle (indigenous boat) for a living, transporting people from one bank to the other. Malaichami and Kuyil become good friends despite the age gap and the vast difference in their social and financial status. Since Malaichami gets no emotional support or love at home, he enjoys Kuyil's company.
Chellakannu falls for Selvi (Ranjani), daughter of a cobbler (Veerasamy) and they plan to elope when Selvi's wedding arrangements are being made. On learning this, Malaichami reprimands Chellakannu. Kuyil intervenes and convinces Malaichami to get them married. After the marriage, when Chellakannu and Selvi are alone and playing in a field, Selvi is killed by someone. Before she dies, she bites the big toe of her assailant. Initially, the villagers think it is murder for gain, but when Malaichami sees Rasamma's husband coming home without a toe, the puzzle is solved and he is handed over to the police. Ponnatha gets angry at this and alleges he did this because he is not the biological father of her daughter. When the villagers discuss Malaichami and Kuyil in hush-hush tones, Ponnatha gets all the more agitated and drags Malaichami to the Panchayat, asking for a solution. Malaichami confesses that he was married to her only to save her from disgrace, that they have no bonding between them, and that he wishes to be with Kuyil; then he goes to meet Kuyil.
By the time he reaches her hut, Kuyil has been arrested by the police for having killed a person in her coracle. She remains silent and does not reveal the motive for the murder to anyone.
When Malaichami visits her in prison and pleads with her to tell him the truth so that she can be rescued, she reveals that the person she had killed in the coracle was Ponnatha's lover who is the biological father of Rasamma. He was on his way to the village after serving a life term in prison and was planning to lay claim on Ponnatha and her wealth. As this would affect the reputation of Malaichami's family, she killed him. Malaichami is spellbound by her sacrifice and takes a vow that he will not die until Kuyil returns and chooses to stay in, her hut. When he is dying, Kuyil is brought to him on parole by the police on a special request. Malaichami dies after meeting Kuyil in the hut. While returning to prison, Kuyil dies on the train. Their epic love story ends.
[from the book Pride of Tamil Cinema: 1931 to 2013 by G Dhananjayan, Blue Ocean Publishers 2014]