Ramanujam Iyengar alias Ambi is a straightforward consumer protection advocate from Triplicane, Chennai. He expects everyone to follow the law and prosecutes those who violate it. However, his efforts fail as circumstantial evidences always seem to favour the accused. His efforts to raise civic awareness go in vain due to pervasive corruption and a general lack of seriousness. Frustrated at his inability to bring about a change in society, his suppressed anger manifests itself in an alter ego named Anniyan, a grim reaper-themed vigilante who murders corrupt and indifferent people. Anniyan creates a website, compiles a list of wrongdoers in it and kills them using punishments described in one of the ancient Hindu scriptures Garuda Puranam.

Ambi is secretly in love with his neighbour Nandini, a medical student and an aspiring Carnatic singer, but never expresses his feelings due to fear of rejection. When he does get the courage to do so during the annual Tyagaraja Aradhana with the help of his friend, Sub-inspector Chari, she rejects him as she cannot bear his overbearing nature and constant complaining and nitpicking. Distraught, Ambi attempts suicide, almost drowning himself before having second thoughts. Subsequently, he develops another personality named Remo, a metrosexual fashion model. Nandini is smitten by Remo and falls in love with him, unaware that Remo is one of Ambi's split personalities. Their marriage is eventually fixed.

While purchasing a plot of land for her dowry, Nandini decides to undervalue the property to evade stamp duty. Ambi, who accompanies her to the government office, refuses to help her on learning of her intention to skirt the law. Later, when Nandini and Remo are on a date, Remo transforms into Anniyan and attempts to punish her for her corrupt act. As he is about to kill her, Nandini calls out for Ambi. Anniyan then reverts to Ambi, who collapses and loses consciousness. Nandini takes Ambi to NIMHANS where he is diagnosed with MPD. Through RMT, Vijaykumar, the chief psychiatrist of NIMHANS, uncovers Ambi's past. It is revealed that when Ambi was ten years old, he witnessed the accidental death of his younger sister Vidya due to civic apathy.

The incident left a deep emotional scar, which is 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 oblivious to their existence within him. Vijaykumar declares that Remo will cease to exist if Nandini reciprocates Ambi's feelings, but Anniyan will cease to exist only when the society reforms. Nandini accepts Ambi's love and Remo disappears.

Meanwhile, DCP Prabhakar and Chari investigate the murders committed by Anniyan. In disguises, they discover clues left behind by Anniyan, which are the names of the punishments he meted out to his victims. Prabhakar is personally determined to punish Anniyan, as one of his victims, Chockalingam, an errant catering contractor with the Indian Railways, was his elder brother. In a dramatic publicity stunt, Anniyan admits to the murders he committed when he appears amidst the public and the press at the Nehru Stadium. He explains the rationale behind them and says that only when every Indian is responsible and sincere will the country prosper on par with developed nations. His methods draw both praise and criticism. Prabhakar tries to catch Anniyan, but he escapes.

On investigating the phone records and IP address of Anniyan's internet activity, Prabhakar discovers that the connection belongs to Ambi. Upon further analysis of a security footage of Anniyan's public appearance, Ambi's face is revealed under the hood. Prabhakar arrests Ambi and subjects him to a polygraph test to make him confess to his crimes. The officers see that he is telling the truth and is oblivious to killing anyone, believing his innocence. Prabhakar, enraged and not having any of it, orders all the officers and Chari to leave, and begins brutally attacking and tormenting Ambi alone with Garuda Puranam punishments. Ambi is eventually thrown into a boiling hot salt water tank by Prabhakar.

When a traumatised Ambi begs for mercy, Prabhakar mockingly turns on the water cooler, making the water extremely cold to the point it becomes ice so that Ambi will freeze to death. The near-death experience and pain triggers Anniyan's reappearance. Ambi's personality alternates between Ambi and Anniyan; he brutally subdues Prabhakar as Anniyan, while begging for mercy as Ambi. Later, when Ambi is tried for several murders, Vijaykumar testifies to Ambi's mental condition. Chari also had secretly recorded Ambi's interrogation and torture at Prabhakar's hands and presents it as evidence of Ambi's mental condition. Ambi is acquitted, but is directed to undergo psychotherapy in a mental hospital and will be eligible for release when cured.

When Ambi is released two years later, his rigid adherence to protocol has diminished and he appears more open and accommodating. He marries Nandini. and while travelling on a train during their honeymoon, Ambi notices a man (an electrician who was indirectly responsible for his sister's death several years ago) drinking amidst fellow passengers. Moving himself and Nandini away from the man, he surreptitiously makes his way back to the compartment and throws the man off the train, killing him. However, he hides the incident from Nandini, revealing that instead of Ambi eradicating his other personality, both have blended into one identity.