Anjanam Azhagiya Pillai (S. J. Suryah) is an ardent devotee of Vallalar since childhood, and hence, he respects each and every living creature from very young age. Anjanam, aged 36, lives alone in Chennai, although he is an engineer in the state electricity board with good pay. He has two major goals in life - to get married soon and to own a good house. After the 150th marriage rejection, he buys a flat in a beautiful apartment. As soon as he moves to his new flat, Anjanam gets the nod from the former bride Meghala's (Priya Bhavani Shankar) side. Though the new house turns out to be lucky for Anjanam, he cannot sleep peacefully because of a naughty rat which turns his house and life upside down. Despite all of Anjanam's effort, the rat keeps on returning to his home and troubles him. Meanwhile, a diamond smuggler named Sarath (Anil Kumar), who lived in Anjanam's house before him, comes in search for his hidden diamonds, which he left in a hurry while vacating. A month later, he finds that the rat had ingested the diamond and wants to cut the rat open to get the diamond. The smuggler disguises himself as a pest and rat control guy and comes to the aid of Anjanam. The smuggler hires a rat specialist and tries to kill the rat by many methods, such as by using like poisonous smoke and poisonous food.

That night, Anjanam gets a very disgusting smell in his house that makes him believe that the rat is dead. The next day, Anjanam finds out that the rat is not dead and tries to kill it with his help of his friend Ravi (Karunakaran). In that chaos, he finds out that the rat did not eat the poisoned food, but the neighbor's cat did. The cat was found in a decomposed stage with grubs on it. The rat is captured finally, and the smuggler drops it inside a bucket of water to drown. However, Anjanam saves it as he is against the murder of any living thing. The smuggler and Anjanam fight for the rat's life, and Anjanam saves it by letting it free. An hour later, Anjanam recaptures it, takes it out 10 km away, and frees it. Anjanam gets a call that Meghala, who was working in a diamond store, has been arrested by police for stealing a diamond. When Anjanam reaches the police station, the inspector clarifies saying that the CCTV camera shows that Meghala is innocent and that the diamond does not belong to the store after all. Hence, Anjanam gets hold of the diamond as a bonus. He plans to leave town for his impending marriage with Meghala and friends. While packing his bags, he hears squeals from his kitchen and finds the rat's babies. He realizes that the rat was returning only to protect its children. He reaches out to the same spot and leaves the rat's children there. The rat happily collects its babies.