Pournami is the story of a village where a tradition is being followed since ages. A dancer had saved the village from the drought ages back by dancing in Shiva`s temple for days and nights. Her last wish was that every 12 years a girl from her family must perform the dance on the night of Pournima.
Then comes the present day when the elder daughter of the family, Pournami (Trisha), goes missing. Rumors are that she has eloped, much to the dismay of her father who shall not teach the dance to her younger sister, Chandrakala (Charmy).
Their step-mother now rules the house and rents away a room to Siva Kesava (Prabhas), a Western-style dance instructor.
The rest is about Chandrakala and Siva`s blossoming relationship and how Siva defeats the villain and revives the ritual by teaching her the dance.