An alcoholic professor is sent to a juvenile school, where he clashes with a gangster who uses the school children for criminal activities.
2002: In Nagercoil, a 17-year-old Bhavani witnesses his family's murder by his father's rivals. He is forced to take the blame and is incarcerated in a juvenile detention facility, where he is tortured mercilessly. As a consequence, he develops a ruthless and brash persona. Years later, Bhavani is released from the facility and becomes the owner of the lorry and trade business. Gradually, he becomes a feared gangster and remains immune from punishment by coercing the inmates of the detention facility to take the blame for his crimes. Creating an enormous syndicate for himself, he grows on par with his father's former rivals and eventually kills them.
2019: In Chennai, John Durairaj (J.D) is a college lecturer beloved by his students for his student-friendly approach. He is struggling with alcoholism and bitter dislike from fellow staff. Charulatha aka Charu, a newly hired lecturer at his college is the only one who sympathizes with him. After J.D organizes a college election that goes awry, he is assigned to a juvenile detention facility for a 3-month assignment he does not remember volunteering for. J.D reluctantly assumes his new position and is warned by the prison authorities of the rampant drug abuse and violence there, but J.D pays no heed. Sabari and Manikandan are two brothers, who are supposed to surrender for one of Bhavani's murders, slip a note to an unknown J.D, and later that night try to call him while he is drunk. When they are caught, Bhavani kills them and their bodies are discovered hanging from the ceiling of the detention center classroom the next day.
J.D is detained for the murders. A grief-stricken Charu admonishes J.D for his negligent behavior, revealing that she was the one who had signed him up for the job, believing him to be the only teacher capable of understanding the needs of the inmates. She reveals herself as an NGO worker investigating the crimes occurring in the facility; the brothers had desperately contacted her for help. While in detention, J.D. discovers the brothers' note, detailing the horrific activities taking place inside the center. Heartbroken, he clobbers Bhavani's goons who had been dispatched to kill him and calls Bhavani, promising to make him pay for the boys' deaths. Having quit drinking, J.D slowly gains control over the institution and begins to reform the inmates. His actions are opposed by Das, an older inmate (found to be an adult) responsible for controlling the younger children.
With the assistance of a young inmate, Undiyal. J.D discovers that Bhavani regularly makes the inmates take the blame for his crimes. He records the video of the younger inmates including Undiyal, speaking about Das's crimes in the facility. Bhavani's goons target Charu and retrieve the video from her, sparing her only because of Undiyal's intervention. Undiyal reveals that select inmates are tattooed with the Tamil letter Pa, which allows unrestricted access to free food and drugs from Bhavani's associates in exchange for surrendering for his crimes. J.D raids the respective establishments and cuts off the drug supply to the detention facility, much to Bhavani's frustration. He also sneaks into Bhavani's hideout and threatens him and his men with dire consequences, while unknowingly holding Bhavani at knifepoint using a ball pen. Bhavani retaliates by killing some of J.D's batchmates and ex-students.
With J.D incapacitated by grief, Das abducts most of the inmates, intending to take them out of Tamil Nadu along with Bhavani's Benami documents. With the help of his friend and expert archer Vanathi, J.D defeat Das's men and rescues the children. J.D confronts Das and makes him realize Bhavani will kill him because he is the link between Bhavani and the crimes in the detention facility. Das reforms and transports J.D to Bhavani's meatpacking plant along with the children. Das overpowers the men sent to kill him while J.D confronts Bhavani. Although Bhavani gets the upper hand, J.D manages to defeat Bhavani, who later urges J.D to join him in politics, but J.D kills him, thus avenging Sabari and Manikandan's death. With the children safe, J.D and Das surrender to the police. The detention facility is shut down by the government, with the inmates being sent to other detention facilities and state prisons, based on their age. J.D., Das, and the other adults from the facility are sent to prison, where they intend to reform the inmates there.
