Ratsasan

Two elderly men discover the corpse of Samyuktha, a 15-year-old schoolgirl, at the start of the investigation. The brutality signals the rise of a serial killer operating in the area. Arun Kumar learns about the case and its disturbing implications for the community.

Arun Kumar, an aspiring filmmaker fascinated by psychopaths, becomes a Tamil Nadu police sub-inspector under family pressure and with help from his brother-in-law, Doss. He then moves in with his sister Kokila, Doss, and their daughter Ammu. The new role places him at the center of the evolving investigations.

An incident at Ammu's school brings Arun into contact with her teacher Viji. He befriends Viji and becomes close to her hearing-impaired niece Kayal. This alliance ties Arun more closely to the cases surrounding the students.

Arun investigates the abduction of Amudha and notices a mutilated doll at Amudha's house. His theory about a linked killer is dismissed by his superior ACP Lakshmi. Soon, Amudha's corpse is discovered, and a pathologist confirms the same brutal modus operandi.

A few days after Amudha's case, another hearing-impaired girl, Meera, is abducted and murdered. The investigation now points toward a serial pattern matching the earlier killings. Arun tracks similarities to the Meera murder and gains new clues.

The investigation leads to Inbaraj, a teacher at Ammu's new school, who is a pedophile. He is caught after a victim tips Arun and confesses to sexual abuse, though he denies involvement in the murders. During an attempted escape, Inbaraj is shot dead by Arun. On the same day, Ammu is abducted from her birthday party and her mutilated body is found in a car trunk.

Arun is suspended for his actions, but he continues investigating unofficially. With help from a few officers, he traces a piano tune from Meera's hearing aid to magician Annabella George, who had performed at all the victims' schools. This clue begins to unravel a hidden perpetrator.

Arun learns that Annabella randomly selected students during her shows, abducting and killing them. The next target becomes a girl named Sanjana. Despite surveillance, Sanjana is abducted, but Arun and his team manage to rescue her, though Annabella escapes.

Arun meets retired cop Rajamanickam, who reveals Annabella's true identity as Mary Fernandez. He explains that her son Christopher suffers from Werner syndrome and is a gifted magician and pianist who was mocked at school. Sophia, a girl who befriended Christopher, was murdered, and Mary and Christopher were arrested but presumed dead in an accident.

Rajamanickam warns Arun that Mary Fernandez may still be alive and holds information that could expose the truth. Before he can divulge more, Rajamanickam is killed, cutting off the lead.

Arun finds clues suggesting that Christopher is alive and has been impersonating Mary. Before Arun can inform Lakshmi, Christopher launches an assault on Viji and abducts Kayal. The hunt intensifies as Arun races to stop him.

Christopher kills Venkat and Dr. Nandan to silence those who could expose him. Arun tracks Christopher to a final confrontation, engages in a violent fight, and manages to kill Christopher. Kayal is rescued, though Arun is injured.

With the killer dead and the case closing, the media reports the end of the serial murders. Arun is finally recognized for his insights and gains a chance to make a film about a psychopath, fulfilling his dream.

The investigation leaves a lasting impact on Arun's career and personal life. He steps into the limelight as a creator who can translate real-life psychopathology into cinema. The community is left to process the trauma and the revelations about the killer's identity.