Duraisingam is an honest SI of Nallur, a small village in Thoothukudi district in southern Tamil Nadu, assisted by his bumbling colleague Yettu Erimalai. He belongs to Nallur with his father Soundarapandi, having a respectable status in the village. His family business is provision stores, and Singam wants to join it, but he joined the police due to Soundarapandi's wishes. He resolves most of the problems in his village with non-violence and mutual counselling. He uses force only when the situation demands it, thereby gaining much reputation and love from the villagers.

Mahalingam, an industrialist in Chennai and friend of Soundarapandi, comes to the village with his daughters Kavya and Divya (Krishna Priya). Singam initially thinks of Kavya as a tiger when she is about to prank her cousin by wearing a tiger costume. As Singam is roaming around the village, he accidentally slaps her. As Kavya is about to take revenge on Singam, he apologizes to her. Kavya gets moved by this and slowly falls in love with him. After some hilarious incidents, Kavya professes her love to Singam. Initially taken aback, Singam soon reciprocates Kavya's love.

Mayil Vaaganam is a Chennai-based big-time extortionist with shady mafia dealings who blackmails people by complaining to the Income Tax Department, is required to travel to Nallur to sign a conditional bail. Instead, Mayil sends one of his allies to do the formalities, much to the anger and rage of Singam, who demands that Mayil sign the bail in person. Humiliated, Mayil reaches Nallur but is unable to take any revenge on Singam, fearing the immense love and devotion of the entire village towards Singam. Using political contacts, Mayil Vaaganam gets Singam transferred to Chennai to teach him a lesson. Singam, unaware of Mayil's hand behind his transfer, joins Thiruvanmiyur police station. His coworker SI Ravi hates Mayil for his crimes, but is unable to take any action because of Mayil's political powers.

Rajendran, Singam's senior is on Mayil's payroll and takes care in concealing and eliminating the evidence of Mayil's crimes from the eyes of the law. The Commissioner also does not help Singam as there is no evidence against Mayil, and Rajendran warns Singam to stay away from Mayil's case. Unable to take on Mayil in his stronghold, Singam wants to return to his village, but is stopped by Kavya, who encourages him to fight against Mayil and end his crime network. Being mentally tortured by Mayil, Singam arrests Mayil's brother Vaikuntam in a fake case of illegally smuggling alcohol. He thwarts off Rajendran in full view of the public when Rajendran, bounded by his duties to Mayil, tries to protect the henchmen. Mayil kidnaps Divya for ransom.

Singam rescues her with unexpected help from Home Minister Ramanathan. Singam successfully traces the origins of the kidnapping racket to Mayil and also gets promoted to ACP of the specially-formed Anti-Kidnapping Task Force. Mahalingam, who was hostile to Singam following an altercation with Soundarapandi back at Nallur, softens up and agrees to give him Kavya's hand in marriage. The entire police force, including the Police Commissioner and Rajendran, are now on Singam's side. They decide to help him fight Mayil. They manage to kill Mayil's henchmen in an encounter at a hospital and begin to target everyone and everything related to Mayil.

In retribution, Mayil starts targeting everyone close to Singam, including Kavya, whom he shoots but is saved by Singam, and Ravi, who is hacked to death by Mayil's henchmen. To escape the arrest warrant issued out to him, Mayil kidnaps Karnataka Home minister's daughter. He falsely tells Singam that he is going to Pondicherry with her, when actually he is going to Nellore in Andhra Pradesh to put the police off the track. However, Singam manages to pursue them till Gudur near Nellore, where he rescues her and kills Mayil in an encounter. After that, he resigns his job publicly in a felicitation function organised for him.

Singam is seen with Kavya heading back to Nallur. He is stopped briefly by Ramanathan, who offers an undercover mission,[N 1] to which he willingly agrees.