Aarusaamy Saamy arrives in Tirunelveli, along with his friend, witnesses a corrupt cop trying to extort money from someone, he thrashes him, which prompts the police to arrive at the scene, and when they question Saamy, he reveals himself to be the new DCP of Tirunelveli city, while ordering the police to arrest the swindler posing as a cop. Saamy later assembles the police force and rules out their procedures that they can do whatever they wish, provided the public and the innocent are not harmed in the process. He also tells them to arrest any thug in the town without second thoughts.

Due to Saamy's administration, the crime rate in Tirunelveli decreases. At the outset, Saamy pretends to be a corrupt cop by accepting bribes from the influential and corrupt Politician Perumal Pichai under the condition that his illegal activities will not be monitored, while advising him to move his liquor stores elsewhere in order to avoid disturbance to the public. Perumal accepts, merely out of an assumption that Saamy belongs to his caste. Saamy falls in love with a college-going Brahmin girl Bhuvana, whose father Srinivasan, is a straightforward government officer who dislikes bribery and leads a noble life.

Saamy and Bhuvana meet each other when Saamy leaves with his assistant, Inspector Punctuality Paramasivam, in search of a home for rent. Bhuvana misinterprets Saamy and Paramasivam as thieves and locks them in a room, only to be revealed of their identities by the police. Soon their frequent rendezvous turn to love. One day, Saamy seals Perumal's gas station because the workers assaulted a woman and others when they challenged them a very less distribution of petrol than being promised. This infuriates Perumal, who storms into Saamy's quarters with Bhuvana being present, and they argue. Bhuvana gets upset when Perumal mentions the bribe Saamy had taken from him, leaving to her home.

The next day, Saamy tries to convince Bhuvana at the temple and reveals his past. Saamy's father Chokkalingam wanted to become a cop, but he was unable to due to corruption. So, he takes care of agriculture as his living, but he wanted to make his son a cop. Saamy passes the TNPSC Group 1 exams, but was asked for bribes. Chokkalingam mortgages his properties and makes him get police job. Being an honest cop, Saamy is honored with transfers all over Tamil Nadu due to political pressure. In Trichy, he is accused of bribery and gets suspended. After 6 months, Saamy proves his innocence, and is finally posted back to Tirunelveli, and adapts a new policy and also taking bribes from the influential persons, for the public's welfare.

Saamy also tells Bhuvana that the bribes are entirely donated by him. Bhuvana reconciles with Saamy, and after much coercion, Srinivasan agrees to their engagement. The ruling party calls for a one-day strike all over the state, and Perumal is to handle with the responsibility of Tirunelveli on the eve of Pongal. The retail vendors plead with Saamy to provide protection as Pongal festival is a very important sales time. Saamy takes steps to maintain law and order. The strike becomes a failure in Tirunelveli, which enrages Perumal. Perumal was waiting for a chance seek vengeance against Saamy for sealing his petrol bunk earlier. So, his henchmen attack the market on the day of Saamy and Bhuvana's marriage, since all the prime policemen would be attending the wedding.

Saamy witnesses the brutality of Perumal's men, and this incident begins their enmity. Saamy receives his transfer order to Dindigul, with 1 week duty remaining in the city. Saamy challenges to close Perumal's empire and finish him in that week, which he succeeds in doing so by using various enforcement tactics. Saamy arrests the main accomplies of Perumal, two of the MLAs and imprisons them for 15 day period. Saamy covertly kills off other main enforcers of Perumal in a riot caused by him during a procession. Saamy advises Bhuvana to leave for Pazhani. In the evening, after listening to Perumal's phone calls, Saamy realises that Perumal has planned to bomb somewhere.

Saamy meets with Perumal's henchman, whom he spared in the rally shooting, and convinces the henchman to turn approver. Saamy finds that the bomb is to be detonated in his house. Unknown to Saamy, Chokkalingam has arrived at his home from Pazhani. Despite Saamy's attempts to warn Chokalingam, the bomb detonates, and Chokalingam is killed. The bomb maker and Perumal's henchman provide statements to the magistrate, and Saamy is provided an arrest warrant against Perumal. Despite hurdles by the authorities and ministers to stop the arrest, Saamy leaves to arrest Perumal.

On the final day eve, Saamy gives news to the media, that Perumal has absconded, and police are searching for him. Saamy routes Perumal to his own sand quarry. After a brief fight, Perumal surrenders to Saamy and scoffs at him. Saamy refuses, kills Perumal with his illicit revolver, and burns his corpse in a brick kiln nearby, so that no one will try to replace or avenge the absconding Perumal, and thus no harm done. Saamy then performs the rites for his deceased father.