Chitra, a medical student and her friends are working on a project about the human brain. She wants to investigate the curious case of Sanjay Ramaswamy, a notable Chennai-based businessman who is reported to have anterograde amnesia. Her professor denies access to Sanjay's records as it is currently under criminal investigation. Chitra, overcome with curiosity, proceeds to investigate the matter herself in secret. Sanjay is suffering from anterograde amnesia; he loses his memory every 15 minutes and uses a system of photographs, notes, and tattoos to recover his memory after each cycle. It is revealed that Sanjay is ultimately out to avenge the death of his lover Kalpana, and that he is systematically killing the people responsible.

Sanjay's main target is Lakshman, the head of a Kolkata-based human trafficking network and a notable socialite in Chennai. Inspector Ravi is on the case of the serial murders, who tracks Sanjay at his flat, knocks him unconscious. Ravi finds two diaries where Sanjay has chronicled the events of his life. Sanjay Ramaswamy is the owner of the AirVoice mobile telephone company, who meets Kalpana, a struggling model, and secretly develops romantic feelings for her, while introducing himself as "Manohar". Eventually, both of them spend time together and gradually develop a liking towards each other. The diary ends with Sanjay proposing to Kalpana and promising himself that he will reveal himself as Sanjay Ramaswamy if she accepts.

Before Ravi can read the 2003 diary, Sanjay regains consciousness and ties him up. He tracks down Lakshman to a college function where Lakshman is the guest of honor. Sanjay takes pictures of Lakshman and decides to kill him. However, Sanjay mistakenly attacks and kills one of Lakshman's goons in the parking lot. Lakshman is perplexed and fails to recollect the incident. He decides to find and kill his enemies one by one, but Sanjay is not among them. In the meantime, Chitra visits Sanjay's flat and finds Ravi, beaten and bound. Chitra finds the two diaries and frees Ravi. Sanjay arrives suddenly; he remembers neither of them and chases them out. Ravi is eventually hit and killed by a bus, while Chitra barely escapes, going into a phone booth.

Believing Lakshman is in danger, Chitra informs him that Sanjay is after him. Sanjay discovers that Chitra had warned Lakshman and goes to her dormitory to kill her, where Chitra calls the police and Sanjay is arrested but later freed. Lakshman discovers about Sanjay through a police inquiry and arrives at Sanjay's flat, destroying all the photographs, notes, and scratching off Sanjay's tattoos. Chitra reads the second diary, which states that Kalpana accepted the proposal, but on the condition that she marry only after she completes her past commitment. The diary ends abruptly as Sanjay left for a business trip. Chitra investigates further and discovers that Kalpana was travelling to Mumbai for a modelling assignment by train when she rescued 25 innocent young Tamil girls being trafficked to Mumbai and Kolkata.

However, Lakshman, who is the ringleader of the racket killed those two girls, who recognized him and goes in search of Kalpana. His goons broke into her apartment and tried to kill her, only to be overpowered and fought off by Sanjay, but Lakshman attacks Sanjay and kills Kalpana in the same way. Now avare of the truth, Chitra finds Sanjay in the hospital and tells him the truth. Sanjay tells her to lead him to Lakshman. Lakshman's twin brother Ram arrives from Kolkata to help him pursue Sanjay. Arriving at Lakshman's lair in downtown Chennai, Sanjay confronts all of Lakshman's henchmen and, with superior strength, manages to disable them. Sanjay fights Lakshman and Ram, overpowering and killing the brothers. While Sanjay drives Chitra back to her dormitory, he stops at a crossing to let young children cross the road. A young girl smiles at him, and he smiles back at her.