Unnodu Ka opens with a clash between two families in Sivaganga. Jayavel (Prabhu) and Keerthy Vasan (Thennavan), the heirs of these families, are expected to take on each other, and thereby safeguard the village's 'tradition'. However, they are actually thick friends, and have no clue about the feud, which has been prevailing for five generations. They leave to Chennai with their respective families in search of a peaceful life, and settle there as neighbours. Jayavel's son Siva (Aari) and Keerthy Vasan's daughter Abhirami (Maya) are brought up together. Interestingly, the two were born on the same day, though Abhi was born just a second before Siva — one of the reasons for her overbearing attitude towards him. Their parents hope they get married to each other, but the two are often found quarrelling. However, they realise their true feelings for each other when they try to help out their friends, Bhagat Singh (Bala Saravanan) and Sundarambal aka Su (Misha Ghoshal).