Maruthu is a rustic action entertainer where Vishal plays the role of a labourer. The story revolves around his relationship with his grandmother.
Maruthu is a labourer from Rajapalayam who does not tolerate injustice and leads a happy life with his grandmother Appathaa, and his close friend Kokkarako. One day, Maruthu encounters Bhagyalakshmi "Bhagyam", a bold young woman, at a temple, but their first meeting goes badly due to a misunderstanding, and he ends up at a police station. With Appathaa's help, Maruthu soon manages to win Bhagyam's heart. One day, Maruthu sees some goons chasing Bhagyam and her father near the market where he is working. Maruthu beats up the goons and then learns from Bhagyam's father why the goons are after them. Bhagyam was the daughter of Mariyamma, a brave woman who stood up to the atrocities committed by the municipal chairman Rolex Pandiyan on the local people. When she decided to run against Pandiyan in the upcoming municipal election, Pandiyan and his men brutally hack her to death and stuff all her parts except her head into a bag.
Bhagyam and her father are unable to prove in court that Pandiyan committed the murder since there is no head and the Inspector is Pandiyan's henchman. They are also threatened with dire consequences by Pandiyan if they continue to target him. Appathaa, who witnessed Mariyamma's brutal murder and is upset that she didn't stop it, gets Maruthu married to Bhagyam to repent her earlier actions, and Maruthu also promises to avenge Mariyamma's death. A cat-and-mouse game soon begins between Maruthu and Pandiyan, with Pandiyan using all means to stop Bhagyam and Appathaa from testifying against him, but to no avail. At the court, Appathaa testifies against Pandiyan, who is issued an arrest warrant against him. In retribution, Pandiyan kidnaps Appathaa and brutally tortures her to death. Enraged, Maruthu single-handedly fights Pandiyan and his gang and kills them all.
