Ambi aka Ramanujam Iyengar (Vikram) is an innocent and honest lawyer hailing from an orthodox Iyengar family. If he comes across anyone not obeying the law, he files cases against them. His regular habit to be perfect in rules and honesty leads him to develop a Multiple Personality Disorder in which a new personality in him grows as "Anniyan" (Vikram), a grim reaper-themed killer. Anniyan is violent and deranged, and Ambi has no clue that such a monster exists within him. Anniyan kills lawbreakers by methods used for torturing people in hell, as mentioned in the Garuda Purana. He then indicates the Sanskrit name for the punishment used at the place where he kills them. Some of these punishments are-
Anthakoopam- A man (Cochin Haneefa) refused to help an old man in a critical condition. Anniyan forces a herd of buffaloes to stampede him to death.Kumbipakam- A railway catering contractor failed to provide decent food for the railway passengers. Anniyan tosses him into a pot of oil and fries him to death.Krimiibhojanam- A brake factory owner (Saurabh Shukla) sent a group of thugs to kill Ambi, who had filed a case against the factory for supplying defective brakes. While Ambi is being beaten up by the thugs, Anniyan is triggered who overpowers them. Then he ties the owner to a tree and dumps leeches onto his body.DCP Prabhakar (Prakash Raj) and Chari (Vivek), a friend of Ambi, investigate the murders committed by Anniyan. Incidentally, Prabhakar is the younger brother of the contractor killed by Anniyan. Prabhakar decides to avenge his brother's death by pursuing Anniyan. Ambi is in love with his neighbour Nandhini (Sadha), a medical student, who also teaches music. Ambi proposes to her, but she rejects him due to his staunch following of rules. Distraught, Ambi attempts suicide, but is saved when a third personality comes to life within him. This persona, a suave fashion model named "Remo" (Vikram), woos Nandhini, who instantly falls for him.
Later, Nandhini acquires land in Tambaram, but buys it at less than the original amount to avoid paying tax to the government. Ambi is saddened by Nandini's act, triggering Anniyan. Anniyan chases Nandini and tries to kill her, but then reverts to Ambi, who does not know where he is and what he is doing and falls unconscious. A terrified Nandini then takes Ambi to NIMHANS, where he is diagnosed with multiple personality disorder. At NIMHANS, Ambi reveals to the psychiatrist (Nassar) his dark past- when he was 10 years old, he witnessed the tragic death of his 8-year old sister Vidya, due to civic apathy. The psychiatrist concludes that the death of Vidya resulted in the development of this psychological disorder. He also triggers Anniyan and Remo and finds out that while Anniyan and Remo are fully aware that they are Ambi, Ambi has no clue about his multiple personalities. The psychiatrist then comes to the conclusion that while Remo can disappear if Nandhini accepts Ambi's love, Anniyan can only disappear if India is free from all its ills. Nandhini eventually accepts Ambi's love and Remo disappears.
Later, Anniyan reveals himself at a gathering at Nehru Stadium. He addresses the problems faced by India and the punishments he has used to tackle them. He wins the outright support of the people due to his ideals and methods. Meanwhile, Prabhakar and Chari continue their investigation and find out that Anniyan is none other than Ambi. Ambi is arrested and is taken to the police control room, where he is brutally assaulted by Prabhakar in retaliation for killing his brother and is left to die. The near-death experience triggers Anniyan, who beats up Prabhakar and four other officers severely, leaving them badly injured. Prabhakar turns up in a wheelchair to Ambi's trial, where it is revealed to all that he suffers from multiple personality disorder. Ambi is sentenced to two years of psychological treatment in a mental hospital after which, if he is cured, he will be released.
Two years later, Ambi is released and he marries Nandhini. By now, his puritan adherence to protocol has diminished. While travelling on a train during their honeymoon, Ambi notices a man (the person who was responsible for ambi's sisters death) drinking in front of women. He suffers a relapse on seeing this, transforms into Anniyan and kills the man by throwing him off the train. However, he hides the crime from Nandhini, proving that instead of eradicating Anniyan, he has become successful in blending the personalities of Ambi, Anniyan and Remo into one.
