Madhan Kumar (Silambarasan) is an auditor by profession, who lives in Chennai and also learns music part-time in a college. Mythili (Jyothika), a naive woman, also learns music from the same college. She gets scared upon seeing Madhan Kumar one day because she happens to witness a dream of her getting raped by him. Later on, she realizes his kind nature and they both become friends.

However, Madhan Kumar also has an arrogant and psychotic personality under the pseudonym Manmadhan, through which he hunts and kills morally corrupt girls in the city by seducing them before rendering them unconscious using chloroform. He burns the girls' corpses and stores their ashes in bottles with their respective names. Whenever he intends to kill a girl by seducing her, he suffers a nosebleed. The media attention turns towards the case of missing girls, and Chennai's new ACP Deva (Atul Kulkarni) takes the charge to find the person behind the crimes.

One day, Mythili finds Madhan Kumar riding a bike with a girl as pillion. On the next day, Mythili gets shocked to see the news that the girl whom she spotted with Madhan Kumar has gone missing. Mythili thinks that Madhan Kumar is behind the crimes in the city and informs Deva about Madhan Kumar's whereabouts. Thus, Madhan Kumar gets arrested.

During the investigation, Madhan Kumar reveals that it was none other than his younger twin Madhan Raj (also Silambarasan) responsible for killing erotic and corrupt girls under the pseudonym Manmadhan. The story then moves to a flashback where Madhan Raj and Madhan Kumar were affectionate with each other. They lived with their maternal uncle Puncture Pandiyan (Goundamani) in Madhampatty in the outskirts of Coimbatore district, as both of their parents had died years ago. While Madhan Kumar was brave, intelligent and practical, Madhan Raj was innocent and sentimental. While Madhan Kumar went to study for auditorship in Chennai, Madhan Raj moved to Coimbatore to study in an engineering college. There, he became very close and friendly with his hostel roommate Bobby (Santhanam), and fell in love with his classmate Vaishnavi (Sindhu Tolani), who also reciprocated his feelings upon seeing his good nature. However, Madhan Raj's senior friend Ravi informed him that Vaishnavi was having an illicit affair with her relative Seenu, who also studied in the same college. He did not trust him and in anger he beat Ravi. When Madhan Raj confronted Vaishnavi, she got angry and thought that he did not trust her.

Madhan Raj realised his mistake and went to Vaishnavi's house to apologize, only to find her having sex with Seenu and enjoying the moment. He realized that Ravi was correct right from the beginning. Raj also heard Vaishnavi's conversation with Seenu that she pretended to love him and would like to marry him so that she could take advantage of his innocence and sincere love, which would help her lead life the way she wanted, and have Madhan Raj as a slave for her. Furious, Madhan Raj killed both Vaishnavi and Seenu. He returned to Chennai and arrogantly narrated the whole incident to Kumar. Madhan Raj decided to change his appearance and set out in search of girls cheating in the name of love to kill them and warned Madhan Kumar that if he informed the police of his whereabouts, he would kill him too.

Coming back to the present, the police investigate about Madhan Raj and get solid evidences against him. Madhan Kumar, with the help of Pandiyan, is released from the case. Mythili apologizes to Madhan Kumar and also proposes to him, which he rejects, saying that he does not love her and had treated her as friend. Mythili leaves the place saying that she will wait for Madhan Kumar, believing that he will accept her love someday.

The story again moves to a flashback, where it is shown only to the audience that Madhan Kumar was indeed the real Manmadhan, and Madhan Raj was innocent of the crimes. After killing Seenu and Vaishnavi, Madhan Raj actually met Madhan Kumar but didn't fight him and felt that he regretted killing them and he did it only because he was triggered due to what they were doing and committed suicide in front of Madhan Kumar's eyes, despite the latter's pleas. Madhan Kumar hence decided to avenge Madhan Raj's death by killing girls who cheat in the name of love, under the pseudonym Manmadhan and developed his "nosebleed" from then on. Madhan Kumar has hidden his brother Madhan Raj's death, thereby using his identity as a scapegoat so that he could escape if caught.

It is shown that Madhan Kumar indeed loves Mythili for her innocence and good demeanour, but cannot and will not express his feelings for her. He also exclaims that if his brother had loved a girl like her, he would also have lived happily and he would have married Mythili and be happy. Hence, Madhan Kumar concludes that avenging his brother's death is more important, and only God can judge his actions. The film ends with a message that even if Manmadhan has escaped the clutches of the police, he would answer for his actions before the law someday.