Ratsasan (pronounced Raatchasan, English: Demon) is an Indian Tamil-language psychological thriller film directed by Ramkumar.
Arun is an aspiring movie maker, he is keen on making a psycho thriller movie for which he goes around meeting producers and reciting the story, but is turned down as producers feel his story is too dark or want more commercial elements. Since his father (an police office) is dead while on duty a while back, his uncle (a police officer himself) convinces Arun to apply in police department.
He clears the exams and finally joins the Police force and comes across a abduction and murder case, the psycho killer aspects in the execution of the murder strikes him hard and he starts to dig deep into the case. He successfully links it with another one of same execution style and explains the similarities to his superiors and is assigned to pursue the culprit.
Meanwhile he falls in love with Viji, a teacher working in Ammu's (his uncle's daughter) school, who helps him catch a child molester Inbaraj whom Arun caught red handed trying to molest his cousin Ammu, inbaraj sustains major injuries from Arun and is admitted in hospital. The police now start to suspect Inbaraj of the murders, knowing this he tries to escape from the hospital and is unfortunately killed by Arun in an encounter. Things take a ugly turn when Ammu is abducted and murdered the same way.
While discussing the events with Viji, they discover that one of the victim had a hearing impaired device which Viji's sister's daugher Kayal also possess and get a clue from it, the hearing impaired device of the victim Meera is recovered from evidence and they find a person playing a peculiar piano tune in the background. Using their influence in the police department, Arun organises a live radio show asking listeners the whereabouts of that piano snippet. Surprisingly one caller identifies the audio and mentions that it was played during an opening act of a magic show that took place in their school annual day function. This becomes a major clue and everyone starts investigating school's where other girls went missing from and they find a recurring link between the events, the magician and the brutal murders.
They track the next such magic event and follows the girl(Sanjana) who gets abducted and find the hideout of the killer, Arun saves Sanjana, but the killer flees. Arun gets reminded of a picture in that place from his serial killer archives he collects and uses in order to write screenplay for his movie, which leads him to find the killers identity. From the information provided by the area inspector (Radharavi), he gets to know that the killer is a young man named Christopher with fast ageing disease who was bullied and mocked at in school due to his disfigured appearance. He was in love with a girl but she rejects his proposal, due to this he becomes more psychotic and with the help of his mom murders the girl. Both were arrested and released from prison after some years.
Being nearly caught, Christopher gets very imbalanced and starts tying all loose ends, he kills the area inspector who arrested his family, he later goes to Viji's house, Viji escapes by locking herself in a room but he abducts Kayal, a helpless Viji informs Arun who is now in pursuit, Arun tracks down with a help of his friend Venkat, who is killed by Christopher while making arrest. Arun arrives to the spot, ensuing a fight killing Christopher and ending this once and for all.
