Ramanujam Iyengar alias Ambi (Vikram), is an orthodox Brahmin and a straightforward consumer protection advocate who expects everyone to follow rules and files cases against those who break the law. However, his efforts fail as the circumstantial evidence always favour the accused. His efforts to raise civic awareness among the public fail, due to their lack of seriousness and pervasive corruption. Ambi is frustrated at his inability to bring about a change. Suppressed over a period of time, his anger manifests itself creating an alter-ego named "Anniyan", a violent and deranged grim reaper-themed serial killer who punishes indifference towards social commitment. Anniyan creates a website and sends out postal cards to random people urging them to file complaints against violators. He compiles a database of wrongdoers from his site and executes them in succession using punishments written in the Garuda Purana. On receiving a card, Ambi lodges the details of the perpetrators and moves on with some hope.
Ambi is secretly in love with his neighbour Nandini (Sadha), a medical student and an aspiring carnatic singer, but never expresses his love due to fear of rejection. When he does get the courage to propose her during the annual Tyagaraja Aradhana at Thiruvaiyaru, she rejects him outright as she cannot bear his strict adherence to rules. Distraught, Ambi attempts suicide, almost drowning himself before ultimately changing his decision. Subsequently, he develops another personality named "Remo", a fashion model. Nandini is smitten by Remo and his personality and falls in love with him. Soon, Nandini's family arranges for her marriage with Remo.

While purchasing a plot of land for her dowry, Nandini decides to undervalue the property to evade stamp duty. Ambi, who accompanied her for registration, refuses to help her. Later, when Nandini and Remo are on a date, Remo transforms into Anniyan and attempts to punish her for corruption. When he is about to kill her, an appalled Nandini calls out for Ambi. With a need arising for both his personas simultaneously, a confusion sets in and Ambi collapses and loses consciousness. Nandini takes Ambi to NIMHANS where he is diagnosed with multiple personality disorder. Through recovered-memory therapy, the psychiatrist (Nassar) uncovers Ambi's past to discover that he had witnessed the tragic death of his younger sister Vidya when he was fourteen years old due to civic apathy. The incident left deep scars in his mind and Ambi cites this as the reason for his lofty ideals. It is also discovered that while Anniyan and Remo are aware of Ambi as a separate person, Ambi is completely oblivious to such personalities within him. The psychiatrist declares that Remo will cease to exist if Nandini accepts Ambi's love, but Anniyan would cease to exist only if the society reforms. Nandini accepts Ambi's love and Remo disappears.

Meanwhile, DCP Prabhakar (Prakash Raj) and Sub-inspector Chari (Vivek), who is Ambi's friend, investigate the murders committed by Anniyan. In disguise, they discover clues left behind by Anniyan, which are the names of the punishments he meted out to the victims. Prabhakar is personally determined to bring Anniyan to justice as one of Anniyan's victims, Chockalingam (Shanmugarajan), an errant catering contractor with the Indian Railways, was his elder brother. Following a wide publicity stunt, Anniyan appears amidst the public and the press at the Nehru Stadium, donning face-paint to conceal his identity. Admitting to the murders, he explains the rationale behind them and appeals that only when every Indian is responsible and sincere will the country prosper on par with the developed nations. His methods draw both praise and criticism. Prabhakar tries to catch Anniyan, but he escapes.

On digitally investigating the recorded footage, Prabhakar discovers that Anniyan is actually Ambi and arrests him. Ambi is brutally interrogated by him and is almost killed, triggering Anniyan. Ambi's personality constantly changes between Ambi and Anniyan, resulting in ambiguity. He brutally subdues Prabhakar as Anniyan, but begs for mercy as Ambi. Using a CCTV, Chari records the interrogation and uses it as evidence of Ambi's condition during his trial. Ambi is sentenced to psychotherapy in a mental hospital and would be released when cured.
When Ambi is released after two years, his puritan adherence to protocol has diminished. He marries Nandini and while travelling on a train during their honeymoon, he notices a man (an electrician who was originally responsible for his sister's death several years ago) drinking amidst fellow passengers. Suffering a relapse, he transforms into Anniyan and throws the man off the train, killing him. However, he hides it from Nandini, indicating that he has successfully blended the personalities of Ambi, Anniyan and Remo into one instead of eradicating them.

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