Thunai is an emotionally charged family drama that centers on Dasaratharaman (played by Sivaji Ganesan), a jovial widower and sub-registrar who uses humor as his primary "companion" to navigate life. While he wishes to find a suitable partner for his son, Raghuraman (played by Suresh), the son secretly marries Radha (played by Radha), a woman with a forceful personality. This leads to a persistent "clash of egos" between the traditional father-in-law and the modern daughter-in-law.

In a parallel narrative, Vidhya (played by Saritha), a woman from a struggling background, is betrayed by a deceptive criminal and abandoned by her own family. Finding herself entirely alone, she finds a fatherly protector in Dasaratharaman. However, their friendly bond is maliciously misconstrued by Radha, who spreads rumors of an illicit affair to criticize her father-in-law. This tension eventually fractures the household.

In the climax, to silence the gossip and secure a sense of mutual support, Vidhya suggests that she and Dasaratharaman get married. He accepts her as his "companion," but in a stunning and noble twist at the registrar's office, he reveals he has arranged for her legal adoption rather than marriage. By choosing to accept her as a daughter rather than a wife, he restores her dignity and shames those who doubted the purity of their bond, ultimately reuniting his estranged family.