Thimiru movie is about Ganesh (Vishal), who comes from Madurai to continue his medical course in Chennai. A group of baddies is on his pursuit. Meanwhile Srimathy (Reema Sen), daughter of Ganesh's professor, wants to meet him and thank him for helping her escape from the clutches of some vile elements.
When Ganesh and Srimathy meet they are surprised, for they know each other. Meanwhile, the gangsters on spotting both vow to kill them. Why? Well, it is flashback time.
Eswari (Shreya Reddy) is a shrewish, arrogant 'kattapanchayat' woman. She lends money at usurious rates and then goes after those families that fail to repay. Srimathy’s family falls foul of Eswari. It is Ganesh who helps Srimathy’s folks. Eswari, however, falls for Ganesh’s bravery and boldness. So she wants him to marry her. But the hero doesn’t.
She ends up kidnapping Ganesh's parents and threatens Ganesh to come to them and marry her. However Ganesh thwarts all her plans. Eventually in a freak mishap, Eswari meets her fate. Mistaking Ganesh to be the reason for their sister's death, Eswari's brothers Periya Karuppu (Manoj K Jayan) and Veerasamy (Vijayan) resolve to kill him.
Ganesh, who reaches Chennai for safety, is eventually forced to go back to his old ways to fight the baddies and restore order.