Thai Poosam' revolves around Sivagami (Bhanupriya), a simple village belle. She has a younger brother Velayutham (Baby Shamili) and a snake living with her. A theft takes place in the village temple, in which the jewels of the Goddess are stolen. Sivgami marries the head of this robber gang and vows to transform him into a pious and good man. How she achieves this is what the remaining part of the movie is all about.