Chitra (Nayantara), 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 (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 an instant picture of the man, and writes on it “done”. Sanjay has anterograde amnesia; 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 lover Kalpana (Asin), and that he is systematically killing the people responsible. His main target is Lakshman (Pradeep Rawat), a gangster and notable socialite in Chennai.
A police inspector (Riyaz Khan), on the case of the serial murders, tracks Sanjay down to his flat and attacks and makes him unconscious. 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. She gives an interview in a magazine citing Sanjay as her lover interest. This irks Sanjay; 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 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 the inspector can read the 2003 diary, Sanjay regains consciousness, 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. 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 and killed 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 discovers 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 2003 diary reveals that Kalpana had accepted Sanjay’s proposal and 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, upon which they give chase and subdue her. Sanjay arrives there to meet Kalpana, but notices the group of men grabbing her, and tries to rescue her. He overpowers them only to discover that Kalpana had disappeared amidst the chaos. He makes it to her room, and upon opening the door, she slowly walks towards him, drenched in sweat. Her last word to him is “Po” (Go), a warning for him to leave, as Sanjay looks down to see she had been stabbed from behind. Sanjay is suddenly aware 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, despite already dying from the knife, and Laksman hitting him in the face with the same weapon, before losing consciousness.
Chitra, now aware of the 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 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, exploiting 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.
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 reacts by smiling back at her.