Rani, the daughter of former Chennai police officer Arokiya Raj, tells her father over the phone that she will be back home in Madurai in three hours. As she leaves the telephone booth, she is approached by someone she knows. Rani does not come home. The following morning, Arokiya Raj finds Rani's finger hanging at his door.
Arokiya Raj calls the DCP in Chennai Crime Branch Raghavan, his old friend from Chennai, to help in the investigation of Rani's disappearance. With Raghavan's assistance, the police eventually find Rani's body in the outskirts of the city. The coroner's report states that the killer bisected Rani's body with a surgical knife, proving that the murderer has a strong medical background, and that he also savagely raped her. Six months following the trauma of losing their only daughter, Arokiya Raj and his wife move to New York City to get away from the pain of losing Rani and Rani's wish was to do M.S there .
Three months later, Raghavan learns that Arokiya Raj and his wife have been brutally murdered in New York. Collaborating with the New York Police Department (NYPD), Raghavan leaves for New York to represent the Indian police. On the flight, he reminisces about his late wife Kayalvizhi, who was murdered by a local gang in an attempt to get to him. Once in New York, he begins his investigation with NYPD detective Anderson.
Raghavan stays at a hotel in downtown New York where he constantly keeps his superiors in Chennai updated on the investigation. At the hotel, he meets his neighbour, Aradhana, an NRI. He immediately notices that she is going through a rough patch in her life and one night, worried about her safety, he breaks into her room, only to find her attempting suicide by asphyxiation. He saves her life and the two strike a friendship. Aradhana reveals that her violent husband is filing for divorce after cheating on her.
Meanwhile, Raghavan and Anderson find a connection between Rani's murder in Madurai and a two-year old unsolved murder of an Indian-American girl, Chandana, whose finger was also found hanging in her boyfriend's car. Raghavan's instinct leads them to Chandana's body in a restricted area in the suburbs, found along with three other American girls, raped and murdered in a similar manner. After narrowing down their suspects using flight records, their suspicion falls onto two Indian doctors who studied in New York— Amudhan Sukumaran and Ilamaaran Aanandhan.
Raghavan and Anderson visit Amudhan and Ilamaaran's apartment to question them, only to find it unoccupied. Raghavan and Anderson break into the apartment and find pictures of the victims and the murder weapons. Before they can call for backup, Amudan and Ilamaaran arrive and a violent fight ensues, leaving Anderson shot dead, Raghavan gravely injured and Ilamaaran unconscious. In order to buy time, Raghavan asks Amudhan why such intelligent young men like them would succumb to such psychopathic acts.
Amudhan explains that they had developed a habit of killing from childhood itself. He claims that he managed to keep his violent urges under control for three years. However, the animal inside him was awoken on the night he and Ilamaaran were arrested by Arokiya Raj for assaulting Rani. The inspector on duty had them locked up at a local station for a night with a eunuch who molested them. Thirsty for revenge, both of them left for New York to pursue their higher studies in medicine and went on to kill many local young women. On their trip to India, they killed Rani and then returned to the States. After finding out that Arokiya Raj had moved to New York, they killed him and his wife. Thinking Raghavan will be dead in minutes due to his injuries, they set their apartment on fire and take the next flight to Mumbai to escape the NYPD.
Raghavan makes it out alive by jumping out the window and is hospitalised. Aradhana takes care of him and their bond grows stronger. Raghavan and Aradhana return to Chennai together. During their flight, Aradhana explains that she has a daughter who is living with her parents in Chennai and she feels guilty for being selfish and wanting to commit suicide. As they wait in line for customs, Raghavan proposes to Aradhana, but she refuses, saying she is not ready for another relationship, having just finalised her divorce, and wishes to focus on being a good mother to her daughter first.
Amudhan and Illamaran, evade the Mumbai police at Chatrapati Shivaji International Airport and finally reach Chennai, leaving a trail of bodies in their path. After seeing Aradhana with Raghavan, Amudhan kidnaps her while Ilamaaran tries to get into Raghavan's house to kill him. When the cops arrive, Ilamaaran attempts to flee but is caught by Raghavan after a chase and fight. Trying to negotiate Ilamaaran's freedom for Aradhana's, Raghavan agrees to meet with Amudhan. The final altercation results in Raghavan killing both Amudhan and Ilamaaran and rescuing Aradhana. The film ends with the marriage between Raghavan and Aradhana.