Anniyan is the story of Ramanujam "Ambi" Iyengar (Vikram), a lawyer who suffers from Multiple Personality Disorder. Ambi is an innocent and honest lawyer. If he comes across anyone not obeying the rules he files cases against them but all his efforts go in vain.
His constant habit to be perfect in rules and honesty leads him to develop a Multiple Personality Disorder in which a new character in him grows as Anniyan (Vikram) and starts a website called to hear people's complaints. The new character, Anniyan, is a violent man. When he is Ambi and comes across a person who does not obey rules he makes complaints in his website. When he is Anniyan, he goes and kills them by methods used for torturing people in hell. He apparently takes the definition of these methods from the Garuda Purana.