Naachiyaar, a no-nonsense Assistant Commissioner in Chennai, is assigned to crack an urgent underage rape case. She tracks down the heavily pregnant Arasi, the victim, who is on the run from her uncle and his goons who plan to force an abortion. Although Naachiyaar steps in to protect Arasi, the uncle and his henchmen slip away and vanish into the city’s shadows. Meanwhile, Naachiyaar’s colleague, Feroz, arrests the prime suspect, Kaathu. Kaathu is sent to a juvenile detention facility, where he recounts his side of the story to a sympathetic warden.
Through flashbacks, we learn how Kaathu and Arasi first met when they were teenagers from the slums. Both grow up without schooling, forced to work to support their families. Arasi works as a maid for wealthy families, while Kaathu does odd jobs. One day, while her employers are away, Arasi invites Kaathu into the house, and they impulsively make love. Arasi soon regrets it and pushes him away.
Back in the present, Arasi gives birth to a healthy baby boy, and Naachiyaar lets her stay with her until local authorities can provide a safe place for mother and child. However, DNA tests show that Kaathu is not the father. Naachiyaar decides to keep this truth secret for the moment to let the investigation unfold.
Feroz arrests Arasi's uncle and manipulates him into guiding the police to a fake doctor who conducts illegal abortions. After Naachiyaar threatens him, the fake doctor reveals that a powerful client, a wealthy North Indian moneylender named Thangamani Prabhu, is the man who raped Arasi. The moneylender is so influential that the doctor lies about the abortion's safety, intending to blackmail him later.
Naachiyaar and Feroz head to arrest the moneylender but are recalled to their superior's office because of the suspect's wealth and connections. When Naachiyaar learns she faces suspension, she abducts and thrashes the moneylender late at night and castrates him publicly in the hotel lobby. She faces an internal inquiry and is transferred, as the police fear bad press after her popular stance on the case. She had previously taken the initiative to reunite Kaathu with Arasi. Her husband reveals the truth to Kaathu, but Naachiyaar refuses to disclose it to Arasi.
Kaathu asks Naachiyaar not to reveal the truth to Arasi. He decides that the baby is his, and vows to keep Arasi safe and never let her learn the truth. The story ends with the couple, along with the baby, boarding a bus, ready to start a life together.
