Ethirvinai is all about seeking revenge without the intention of doing so. Avudaiyappan, a big shot, is an antagonist whose son (Dhanush) got caught by the police for handling 20kg of drugs. The police officer Bharani, who arrested Dhanush, asks for a bribe from Avudaiyappan in exchange for releasing his son. Avudaiyappan sells all his property to arrange the money and gives it to Arivu, a car driver, with the wrong intention of handling it to the police officer.

While on the way to the resort, Arivu's car is stopped by two people (Prabha and Jerry) who ask for a lift. During the journey, Arivu explains the tragedy that happened to his mother because of his owner (Avudaiyappan). When they reach their destination, the two people beat Arivu with the backside of a gun and take the money because they are the owners of the drugs that got caught by the police with Dhanush.

A few hours later, Avudaiyappan is informed by his right-hand man (Thangadurai) that the car is not moving on GPS. Thangadurai searches for Arivu and finds him fainted. Later, Thangadurai beats Arivu at Avudaiyappan's house suspecting that Arivu took the money. However, both Avudaiyappan and Thangadurai don't believe him.

Later, Thangadurai finds a phone in the car and asks Arivu about the photo on the phone (which belongs to Prabha). Only then do they believe Arivu. After letting him go, Arivu puts on his shirt and walks towards the car. It is revealed that Arivu took the money. In fact, Arivu shot both Prabha and Jerry with his gun and took the money. With the help of his friend Omar, he buries the dead bodies. Due to his fear of the police, he decides to take the money. However, fate plays a role as revenge against the owner (Avudaiyappan) for what he did to his mother.

Without the money, Dhanush goes to jail, and Arivu escapes with the money."