The film opens with Chitra (Nayantara), a medical student in a college in Chennai, and her friends working on a project about the human brain. She wants to investigate the curious case of Sanjay Ramaswamy (Suriya), a (former) 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, nonetheless, decides to investigate the matter herself.

Sanjay is introduced as he brutally murders a man. He takes a Polaroid picture of the man, and writes on it "done". It is revealed that Sanjay has anterograde amnesia where he loses his memory every 15 minutes. Sanjay uses a system of photographs, notes, and tattoos on his body to recover his memory after each cycle. It is revealed that Sanjay is ultimately out to avenge the death of his sweetheart Kalpana (Asin), and that he is systematically killing the people who were responsible for it. His main target is a man named Lakshman (Pradeep Rawat), a notable social personality in Chennai.

A police inspector (Riyaz Khan), on the case of the serial murders, tracks Sanjay down to his flat and attacks and disables him. The inspector finds two diaries where Sanjay has chronicled the events of 2002 and 2003. The film flashes back to 2002 as the inspector reads the diary. Sanjay Ramaswamy is shown as the owner of the Air Voice mobile telephone company. In the course of his business, Sanjay sends his men to meet Kalpana, a struggling model, about putting up a billboard above her apartment. The owner of Kalpana's advertising firm (Manobala) misinterprets this as a romantic advance, and in view of a possible lucrative Air Voice ad campaign and other benefits, encourages Kalpana to accept the overture. Kalpana thinks of this as an innocent prank that may fetch her better modelling work, and decides to play out the charade as Sanjay's girlfriend. As soon as Sanjay knows of this matter, he decides to confront Kalpana but falls in love with her on first sight. Thus, he introduces himself as Manohar. Eventually both of them hang around 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.

When the inspector is about to read the 2003 diary, Sanjay returns to senses, attacks him and ties him up. He tracks down Lakshman to a college function where Lakshman is the guest of honour. Sanjay took pictures of Lakshman and decides to kill him. He meets Chitra at the function who, after some conversation, decides to befriend him. Later that evening, Sanjay mistakenly attacks and kills one of Lakshman's goons in the parking lot. Lakshman is perplexed and fails to recollect the incident. However 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 the inspector, beaten and bound. Chitra finds the two diaries and frees the inspector. She also finds that Lakshman is Sanjay's target. The inspector tells her that Sanjay is a known serial murderer. Sanjay arrives suddenly; he remembers neither of them and chases them out. The inspector is eventually hit by a bus, while Chitra barely escapes, going into a phone booth. Believing Lakshman is in danger, she informs him that Sanjay is after him. Lakshman arrives at Sanjay's flat to kill him. He destroys all the photographs, notes and scratches off Sanjay's tattoos. Satisfied that he has neutralised Sanjay by wiping out any trace that connects back to him, Lakshman leaves. Meanwhile, Sanjay finds out that Chitra had warned Lakshman and goes to her dormitory to kill her, but Chitra calls the police and Sanjay is arrested.

Back in her dormitory, Chitra reads the diaries. The film flashes fast to 2002 then back to 2003. It is revealed that Kalpana had accepted Sanjay's proposal. This diary ends abruptly. Chitra investigates further, and discovers that Kalpana was travelling to Mumbai for a modelling assignment by train when she came upon 25 innocent young girls being trafficked to Mumbai. She saves the girls who name Lakshman as the ringleader of the racket. Lakshman kills two girls, who recognised him, and goes in search of Kalpana. His goons break into her apartment and wait for her to return. Sanjay arrives there to meet Kalpana. Her last word to him was 'Po' (Go), a warning for him to leave. The goons attack Kalpana. Sanjay intervenes but Lakshman hits him over the head with an iron rod. Sanjay's last vision was to witness Lakshman brutally murder Kalpana with the iron rod.

Chitra, now aware of the shocking truth, finds Sanjay in the hospital and tells him the truth. He flies into a rage and he tells her to lead him to Lakshman. Meanwhile, Lakshman's twin brother Ram (Pradeep Rawat) 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. Finding Lakshman himself, Sanjay fights Lakshman hand-to-hand. However, Lakshman realises that he is no match for Sanjay's strength, so Ram intervenes. As both brothers look alike, Sanjay's memory loss strikes again. Ram, taking advantage of Sanjay's memory loss, beats him mercilessly. Sanjay lies badly wounded on the floor when he sees a vision of Kalpana. With a sudden burst of last minute strength, and with the encouragement of Chitra, Sanjay overpowers both Lakshman and Ram, and kills them both.

The film ends with Sanjay, still experiencing amnesia, driving Chitra back to her dormitory and stopping at a crossing to let young children cross the road. A young girl smiles back at Sanjay indicating the return of normal life.

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