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 the Garuda Puranam, one of the ancient Hindu scriptures.
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.