The story begins in 1965, when a teenage girl discovers the Hell Correspondence, a supernatural method to contact the mysterious entity known as Ai Enma, the Hell Girl. Ai offers a dangerous pact: she can send anyone the user holds a grudge against directly to Hell, but the price is absolute. In exchange for this vengeance, the person who makes the request must forfeit their own soul, ensuring they will also spend eternity in Hell once they die. Driven by pain, the teenager sends her bully to the underworld, but she spends the rest of her life consumed by regret. By 2019, now old and nearing the end of her life, she reveals this haunting secret to her son, Kudo Jin, a journalist. After her passing, Jin chooses to publish her story, bringing the legend of the Hell Correspondence to the public eye.
In the present day, a high school student named Ichikawa Miho attends a concert featuring Maki, a charismatic and rising musical star. During the event, Miho is harassed and groped by a pervert, but she is rescued by Nanjo Haruka, another girl who is deeply obsessed with Maki. This shared experience forms a close bond between the two friends. Later, they attend a performance by Mikuriya Sanae, a beloved pop star. The event turns into a nightmare when a man named Nagaoka Takuro attacks Sanae, slashing her face with a knife. The resulting scars leave Sanae emotionally devastated and distraught over her ruined appearance.
Searching for a way to cope with her trauma, Sanae discovers Jin's article about the Hell Correspondence. She accesses the site and enters the name of her attacker, Nagaoka Takuro. Upon doing so, she experiences a chilling vision of Hell and meets the enigmatic Ai. The Hell Girl provides her with a straw doll bound by a red thread. Ai explains the rules: once the thread is untied, Takuro will be dragged to Hell instantly, but Sanae's own soul will be claimed by the underworld upon her death.
Sanae seeks guidance from Jin, who warns her against the darkness of the pact and encourages her to find a way to move on without resorting to supernatural revenge. However, Takuro sends her a cruel, heartless letter that pushes her past her breaking point. Overwhelmed by rage, Sanae pulls the thread, causing the doll to dissolve and Takuro to be violently dragged into Hell. When Takuro's grieving mother visits the Mikuriya residence to beg for forgiveness, Sanae coldly admits that she was the one who sent her son to Hell. Sanae then attempts to restart her career, with Maki serving as her manager. However, during one of her performances, Ai suddenly appears to claim her. It is revealed that Takuro's mother, in her own grief and desperation, had used the Hell Correspondence to target Sanae. After Sanae vanishes, Takuro's mother informs Sanae's parents of the truth before committing suicide.
Following Sanae's disappearance, Maki organizes an audition to find a replacement soloist. He selects Haruka for the role, but the relationship quickly becomes toxic. Maki forces Haruka to use drugs and manipulates her emotionally. He warns Miho to stay away from Haruka, claiming that she is no longer "pure." Suspicious and worried, Miho visits Haruka's home and witnesses Maki and Haruka together. After Maki leaves, the once-kind Haruka violently attacks her own mother and then beats Miho, ordering her to stay away.
Distraught and believing that Haruka has been brainwashed by Maki's influence, Miho returns to Kudo Jin for help. Jin confirms that Maki is a dangerous individual and shares a shocking voice recording. In the recording, Maki discusses his plan to kill Haruka during a live performance, intending to use her as a sacrifice to "cleanse the world."
In a desperate attempt to save her friend, Kudo and Miho kidnap Haruka and reveal the truth about Maki's murderous intentions. Haruka later informs Maki that Kudo is aware of the sacrifice, though she does not mention Miho's involvement. Kudo, who had hidden a listening device in Haruka's bag, overhears the conversation, but he is quickly discovered and knocked unconscious by one of Maki's henchmen. Kudo is taken to an abandoned building where Maki brutally murders him.
When Miho learns of Kudo's death through the news, she realizes that Maki is a monster who cannot be stopped by conventional means. She attends the final concert and, utilizing the Hell Correspondence, pulls the string of the straw doll. Enma Ai appears once more, dragging Maki down to Hell, allowing Miho to rescue Haruka from the planned sacrifice.
In the aftermath, Haruka and Miho reconcile their friendship. With Kudo's final testimony provided to the authorities before his death, Maki's henchmen are arrested. Maki's true, sinister nature is exposed to the general public, leading to a rapid and total collapse of his fame and a plummet in his album sales.
