Believing her daughter to be Sadako Yamamura's reincarnation, Hatsuko Sofue, Rie Tomosaka, moves to extinguish the threat by locking the child away in a padlocked closet and setting the apartment alight. In a chilling, otherworldly moment, Sadako's ghost escapes a cave on Oshima Island and steps into Hatsuko's apartment, materializing beside the closet as the little girl reappears outside it. The apartment erupts in flames, Hatsuko collapses, and the fire claims several lives, including Hatsuko herself, while the true identity of the girl remains hidden from public record.
At Kurokawa Memorial General Hospital, clinical psychologist Mayu Akikawa, Elaiza Ikeda, begins examining her patient, Ms. Kurahashi, a psychiatric case whose behavior hints at something far beyond ordinary mental distress. Meanwhile, the wandering daughter of Hatsuko, having survived the blaze, collapses after a confrontation with the ghost of a woman who once jumped from a bridge to her death. The police bring the girl to the hospital, and detectives inform Mayu and her supervisor, Dr. Minoru Fujii, Renn Kiriyama, about the fire that killed five people and devastated the apartment—yet there is no public record of Hatsuko's secret ward for the child. The girl explains that her mother called her Sadako, though her real name remains unknown. Mayu notices the girl telekinetically moving toys during the interview, a troubling sign of the otherworldly influence at play.
Mayu's brother Kazuma Akikawa, Hiroya Shimizu, a dropout who has fallen into obscurity, teams up with Yusuke Ishida, a web marketing consultant and aspiring YouTube personality, to rebuild his presence online. Ishida urges Kazuma to film a frightening urban experience to spark interest, and Kazuma agrees to investigate the fire at Hatsuko's building, driven by a mix of curiosity and risk-taking. In the hospital, a mysterious girl is teased by another child, and when she is provoked, she uses telekinesis to hurl a rolling cart, but then collapses after a vision of an old man dying in an elevator. Mayu consoles the girl and learns that she is imprisoned in her mother's closet, a clue that deepens the mystery surrounding the case. Mayu also watches as the well on a lobby TV becomes a conduit for Sadako to exit the screen and climb atop Kurahashi, forcing Mayu to faint.
When Mayu recovers, she discovers the girl is in ICU in a coma, while Kurahashi is readmitted for psychiatric care. Mayu confesses that she has seen a ghostly woman with long hair, and in a dream she re-experiences the girl's imprisonment, a memory that carries the burden of Sadako's origin. Kurahashi challenges Mayu, demanding to know who Sadako is, and Mayu is shown a vision of Kazuma and Sadako herself. Kurahashi warns that Kazuma cannot be saved, and water begins to pool on the ceiling as Sadako's presence intensifies. The ghost emerges from beneath a bed to threaten Mayu, who later awakens to the realization that the girl’s condition has worsened and that Kazuma’s disappearance may be linked to the case. Ishida later reveals a cave on Oshima Island, a shrine for the dead, and a talismanic symbol seen inside the girl's closet, hinting at a broader connection to the urban legend of a cursed videotape.
Mayu connects Ishida's notes to Sadako's origins and concludes that Sadako was born on Oshima Island. The pair travel by ferry to the island, where Mayu warns Ishida that the circumstances surrounding the girl’s upbringing have rekindled Sadako's curse. In a cavern, an old woman explains that unwanted babies were abandoned inside the shrine and that Sadako feeds on their souls. Mayu, who was an orphan herself, wonders if she and Kazuma were among those called to the cave shrine by Sadako's restless spirits. That night, Mayu and Ishida explore the cave; a ghostly hand pulls Mayu through a rock pile while Ishida remains outside. Mayu learns that Sadako's mother, Shizuko, abandoned her daughter, yet Sadako survived and began to wield her deadly power. A vision of the little girl materializes, and writhing bodies rise from the pool to pull the girl underwater. Mayu embraces the child and pleads for her to fight back.
Back at the hospital, the little girl wakes from her coma, and Mayu glimpses skulls and bones beneath the water as the vision dissolves. Ishida finally reaches the cave and finds Kazuma, where Sadako rises to strike Mayu, only to be met with Kazuma's self-sacrificing choice to drown Sadako in the act of saving Mayu. In the wake of these events, Mayu recovers in the hospital and the girl expresses gratitude for the help she received. Yet, as Mayu is alone, Sadako returns to torment her, appearing in her hospital room with the same malevolent focus and extinguishing Mayu's hope in a single, devastating appearance.
