Athiyan, a distinguished Superintendent of Police in Kanyakumari, is widely known as "Vettaiyan," an encounter specialist who effectively neutralizes criminals with the help of his adept team and his unconventional partner, Fahadh Faasil as Battery (Cyber Patrick), a former thief turned tech-savvy police informer with a fondness for Horlicks. The story kicks off when Athiyan receives a crucial tip from Saranya, a middle-school teacher, about the illicit storage of marijuana in classrooms, which has been contributing to drug abuse among students and disrupting their education. Following this lead, Athiyan's team apprehends Kumaresan, the mastermind behind the drug trade, and in a controversial encounter, takes his life, earning public praise yet attracting the attention of the human rights commission. The esteemed Justice Amitabh Bachchan attends the hearing to observe the proceedings, where the court ultimately rules the encounter as justified. Meanwhile, Saranya develops a close bond with Athiyan's family but is soon transferred to Chennai.
Tragedy strikes when Saranya is brutally raped and murdered on her new school's terrace, with her lifeless body found in the water tank. This heinous crime triggers an intense investigation led by SP Harish Kumar and ASP Ritika Singh as Roopa Kiran, uncovering Guna, a software engineer and brother of one of Saranya's students, as the primary suspect. The outrage across the state leads to widespread protests from teachers demanding Guna's immediate arrest and justice for Saranya. After a failed escape from police custody while hospitalized, Guna is eventually tracked down and killed by Athiyan during an encounter at sea, which initially earns him public adoration. However, Justice Sathyadev later reveals Guna's undeniable alibi—he was in Hyderabad during the murder, indicating that Athiyan's hasty actions had resulted in a tragic miscarriage of justice. Distraught over his misjudgment, Athiyan reopens the case with the assistance of Battery, Roopa, and Harish, and they begin to uncover alarming inconsistencies in the initial inquiry.
Their investigation leads them to Hanu Reddy, a hitman hired to eliminate Saranya, but before they can ascertain the identity of the individual behind Hanu's contract, he meets his end. With Patrick's technological insights, the team exposes the corruption of Harish, who had informed Hanu of the police's maneuvers. The inquiry, further augmented by information from Swetha, an employee at the Nat Academy, lays bare a large-scale scam orchestrated by Rana Daggubati as Natraj Shanmugam, who had lured gullible parents and students with outlandish promises of guaranteed success in competitive exams through online courses. Many families took hefty loans, believing in these false assurances, only to face ruthless threats and intimidation when they sought refunds or attempted to withdraw from the programs. To conceal his fraudulent activities, Natraj collaborated with the government under the guise of the "Smart Student Scheme," which misleadingly claimed to offer free preparatory classes in government schools while disparaging their educational standards.
As Athiyan endeavors to arrest Natraj, he faces retaliation with a transfer to the Economic Offences Wing. Undeterred, he teams up with DSP Nazeema to delve into Natraj's dealings under the provisions of the BUDS Act, 2019. With support from his wife Thara, a savvy social media influencer, Athiyan compiles complaints from anguished parents across the state regarding their financial exploitation by the Nat Academy. As Natraj's thugs attempt to obstruct their efforts through intimidation, their plans ultimately fail. Discovering that Saranya had sent a critical hard drive before her death, Athiyan retrieves it, revealing undeniable evidence against Natraj. Tragedy strikes again when Patrick is murdered amid the escalating investigation. Despite Swetha's unethical bribe to the public prosecutor aiding Natraj's evasion from justice, Athiyan perseveres.
Lacking substantial evidence to directly link Natraj to Saranya's murder, Athiyan, with assistance from his nephew, shares Saranya's final video on social media, unmasking Natraj's deceitful operations and asserting Guna's innocence. He then organizes a vital press conference, exposing Harish's corruption and declaring Guna's freedom from guilt. This exposes Natraj, resulting in the termination of all contracts with Nat Academy and its eventual closure. During his transport from Puzhal prison to court, however, Natraj manages to escape while holding Roopa hostage. Athiyan confronts Natraj, ultimately overpowering him and ensuring his arrest once again.
In a turn of justice six months later, Natraj receives a life sentence in prison, while Athiyan, reflecting on his tumultuous journey and missteps, chooses to step back from his prior encounter-driven approach, now embracing the belief that justice should prevail through proper channels.
