Ratsasan

In the opening scene, two elderly men discover a dead corpse of Samyuktha, a 15-year old school student, who was brutally murdered. Arun Kumar is an aspiring filmmaker who wants to make a movie on psychopaths. After multiple rejections and pressure from his family, Arun becomes a sub-inspector of police in  the T.N.P.D, with the help of his brother-in-law, CI Doss. He moves in with his sister Kokila, Doss and their daughter Ammu. Ammu gets into trouble in school and asks Arun for help, so he meets her teacher Viji at school, but later Doss and Kokila catches Arun and Ammu. Slowly, Arun befriends Viji and her hearing and speech impaired niece, Kayal.

Meanwhile, Arun investigates the abduction of a school girl, Amudha. Arun finds similarities between Samyuktha and Amudha's case, due to the mutilations on a doll found at Amudha's house. But his attempts to convince his superior officer, ACP Lakshmi, go futile. A few days later, Amudha's mutilated corpse is found. The pathologist Dr. Nandan, suggests that the perpetrator followed a similar modus operandi  Samyuktha. Over the next few days, another hearing impaired girl named Meera, from Viji's school is abducted and killed. The search leads to Inbaraj, a teacher at the new school Ammu is enrolled. Inbaraj is a pedophile who forces girls into irrumatio. Ammu herself narrowly escapes Inbaraj when Arun nabs him after getting a tip off from another victim Sharmi.

Inbaraj admits to being a sexual predator but denies the murder charges. In a bid to escape, he holds Venkat, a policeman, at gunpoint, but Arun guns Inbaraj down. The same day, Ammu is abducted from her birthday party at home and Arun finds her mutilated corpse in their car trunk. Arun and Das becomes devastated on seeing her corpse. Das didn't tell Kokila that Ammu is dead, and they leaves back to their home. To make things worse, Arun is suspended for his negligence on shooting Inbaraj. Dejected and angry, Arun unofficially investigates the case himself with the help of a few policemen. Finding an audio clip from Meera's hearing aid, he traces a piano tune back to a lady magician Annabella George, who performed at all the victims' schools. Arun finds that Annabella picks students by randomly calling them on to the stage, she then follows, abducts and kills them within the next few days. Arun finds that the next target is a girl named Sanjana. He decides to save her.

Despite constant surveillance, she is abducted. Arun and his team track her location and saves Sanjana. But Anabella manages to escape. Arun finds that Annabella was accused of murder a long time ago. He meets Rajamanickam, a retired cop, to collect information. Rajamanickam tells that Annabella's real name is Mary Fernandez and her son Christopher who has Werner Syndrome (progeria), a hormonal disorder that causes him to appear aged. Christopher performs magic tricks brilliantly and also plays piano well. He is an outcast at school, but a girl named Sophia befriends him.

Christopher starts developing feelings for Sophia and he proposes to her, but he is heartbroken when she turns him down, since his condition has rendered him impotent and he cannot have a love life. Sophie also started avoiding him after this incident. Their conversation is overheard and Christopher is traumatized after many of the boys in his school humiliate him in his school. Mary learns of this and hacks Sophie to death, where she along with Christopher are arrested and sent to prison. They were believed to be dead in an accident, but Rajamanickam says that perhaps, Mary is still alive and has become a psychotic serial killer. Rajamanickam calls Arun to tell something important. Arun reaches Rajamanickam's house and finds him killed by Mary. He sees Rajamanickam's evidences but Lakshmi detains him and forbids him from following this case.

Arun discovers that Rajamanickam had pieced information from photographs and he was going to about to say that actually it was Christopher disguised as Mary, who was long dead in an accident. He tries to reveal it to Lakshmi but fails. Meanwhile, Christopher attacks Viji and abducts Kayal. While trying to confronts Christopher, Venkat is killed. Kayal escapes and is found by Dr. Nandan, who tries to hide her in his facility, but Christopher kills Nandan. Arun tracks down Christopher on time, and after a prolonged fight, he manages to save Kayal and kill Christopher, but he gets injured in the fight. The media finally reports an end to the murders done by Christopher and Arun gets a chance to make a film about a psychopath fulfilling his dream. Then to be continued...