In Unnai Thedi Raghu, studying in Australia, goes to New Zealand with a friend and runs into Malavika. They start off having petty fights but end up falling in love. But circumstances force them to separate without exchanging any information about each other.
Back home, Ajith makes friends with Karan and chances upon his family photograph which surprisingly includes his mother, who as far as he knew had no relations. But on questioning, his mother reveals that she was part of a large family but had been sent out of the family after deciding to wed someone of her own choice. Ajith travels to the village as Karan's friend and soon endears himself to the members of his family. Turns out Malavika is the daughter of one of the brothers too. But when he learns that Karan wishes to marry her, he decides to bow out. But Karan, on knowing about their romance, wishes that they get together.
The family elders too agree to Ajith marrying Malavika but refuse to let the alliance go any further after learning that Ajith is Srividya's son. Ajith too agrees to sacrifice his love since he does not want to wed Malavika and cause even more friction in the already fractured family. How Ajith eventually reunites the separated family and weds Malavika forms the crux of the film.