Near Coimbatore, a brutal murder leaves a distinctive mark on the face—the telltale sign of the so-called “Smile Man.” The case soon intersects with the life of Chidambaram Nedumaaran, who, in the wake of a separate car accident, becomes a central figure in a larger mystery when an unknown assailant targets him. Five years pass, and Chidambaram fights a losing battle with Alzheimer's disease. A doctor warns that his memories could slip away within a year, a prognosis that looms over him as he clings to what remains of his past.
To preserve his experiences, Chidambaram writes a book recounting the serial killings he once investigated, including the infamous Smile Man case, though the narrative remains unfinished. During the book’s launch, he meets Aravind, a CBCID officer who is the son of a former colleague, Venkatesh, who vanished on a mission five years earlier. The encounter rekindles old connections and heightens the sense that unresolved dangers linger. In a bid to uncover the truth, Chidambaram reaches out to Veerayya, who provides information about four convicted individuals who have since been released. He secretly tails them, and after a brief exchange with one of the men, his life is cut short in a murder that mirrors the Smile Man pattern.
The CBCID suspects a serial killer is resurfacing just as Chidambaram’s book enters the public eye. Ignoring medical advice, he joins forces with Aravind to hunt down the psycho killer. As his memory becomes more unreliable, he records conversations and reflections that could unlock the mystery. A supposed copycat emerges—a person who claims to be the Smile Man—but Chidambaram quickly identifies the impostor. Tragedy compounds tragedy when Veerayya is killed by the real killer before Chidambaram can reach him, and the investigation takes a disorienting turn as police arrest an auto driver, mistaking him for the culprit.
A breakthrough comes when CCTV footage from traffic shows the killer’s vehicle, offering a glimmer of direction for the investigation. Overcome with regret for failing to seize the killer sooner, Chidambaram withdraws from the immediate pursuit. Aravind’s inquiry is reignited when a mobile phone found at Veerayya’s house bears Chidambaram’s call, prompting a confrontation that forces Chidambaram to recount a painful past.
In that recollection, Aravind learns that his father, Venkatesh, once led the fight against the same string of murders. Chidambaram had stepped away from the earlier case for personal reasons, only to discover that Venkatesh had closed it by killing an innocent carpenter after his wife mourned a murder. Confronted with this truth, Chidambaram confronts Venkatesh, who tricks and shoots him. Chidambaram retaliates in self-defense, but the moment leaves him stunned by the Smile Man mark on Venkatesh’s face. As Chidambaram speeds away in a jeep, the scene shifts back to the accident that opened the film’s arc.
Aravind’s fury at the possibility that Chidambaram harmed his father drives him to dig deeper, and he uncovers that the 2016 murders occurred near Chidambaram’s own residence. His fiancée, Chithra, and her daughter, Yanu, were victims in the killer’s spree. Chidambaram is placed under house arrest, yet he orchestrates a press conference meant to draw the psycho killer’s attention. Aravind’s investigation leads him to Prabhu, a mortuary worker who emerges as the killer. The suspicion shifts, with Chithram believing Aravind himself could become the next target, while Prabhu murders the wife of a carpenter—someone who had been wrongly implicated by Venkatesh five years earlier—and hangs the body in Aravind’s home, accompanied by a note that another killing remains.
Prabhu’s confession reveals a chilling connection to Chidambaram and the series of killings. He describes a childhood habit of harming friends to force smiles on their faces after death, and his fixation on Chithra—the nurse involved with Yanu—deepens when Chidambaram grows close to her daughter. He manipulates Yanu into pressuring her mother toward marriage, which culminates in Yanu’s death and Chithra’s subsequent suicide. A brutal confrontation ensues as Prabhu attacks Chidambaram, but Aravind and Keerthana intervene, saving him and sending Prabhu to a hospital.
Chidambaram awakens, overhears Prabhu’s recorded revelations, and makes his way to the mortuary to confront the killer once more. Despite his memory lapses, he defeats Prabhu in a final struggle. Aravind then reveals the larger truth: his father’s cover-up had tainted the case, and Chidambaram is the true hero who brought the killer to justice. The film concludes with Chidambaram in a rehabilitation center, still wrestling with fragmented memories of the past, but finally moving toward a quieter, if uncertain, future.
