In 2019, 28-year-old Kim Seo-yeon finds herself in a precarious situation after losing her cellphone while visiting her ailing, estranged mother in a rural hospital. Upon reaching her dilapidated childhood home, she contacts the individuals who discovered her phone, only to discover they intend to extort money from her. To solve her dilemma, she resorts to an antiquated cordless phone she uncovers in the house. The first caller from this phone is a terrified woman pleading for help from her abusive mother, revealing her distressing predicament. Upon exploring further, Seo-yeon stumbles upon a secret basement, discovering remnants from the previous homeowners, which leads her to the realization that she is communicating with Oh Young-sook, a girl who inhabited the same house in 1999.
As they converse, Seo-yeon shares this astonishing revelation about their ability to connect through time via the phone. They exchange poignant details about their lives: Young-sook, orphaned and trapped with her cruel adoptive mother—a shaman—while Seo-yeon grapples with guilt regarding her father's death in a fire she believes her mother Eun-ae caused when she carelessly left the stove on. Young-sook decides to help Seo-yeon by allowing her to overhear conversations from her past, including hearing her father's voice one last time.
Motivated by Seo-yeon's guidance, Young-sook bravely ventures outside to alter the tragic fire’s outcome that claimed Seo-yeon's father. Miraculously, this act transforms Seo-yeon's reality: her parents are alive, thriving, and their home is now splendid—her physical scars have vanished, but Young-sook faces dire repercussions from her mother for her actions, harboring resentment as Seo-yeon's life flourishes while hers stagnates.
Determined to aid Young-sook further, Seo-yeon scours the internet, only to uncover the horrifying truth that Young-sook was murdered by her mother during an exorcism, with her mother subsequently arrested. With time running out, Seo-yeon urgently warns Young-sook of her impending fate, igniting a series of grim events. As the shaman foresees doom for Young-sook, she fights back, ultimately committing patricide.
Meanwhile, in 2019, Seo-yeon enjoys a peaceful dinner with her parents and Sung-ho, the local strawberry farmer. However, in 1999, a now liberated Young-sook succumbs to her dark instincts and viciously takes down Sung-ho when he stumbles upon the harrowing remnants of her crimes. Seo-yeon's life shifts once more, as Sung-ho seems to vanish from her existence, leaving neither parent with any recollection of him.
As Seo-yeon investigates further, she realizes Young-sook's dark path leads to her being identified as a serial killer, and again—during their next call—she inadvertently discloses that Young-sook will surely be caught. Young-sook, inquisitive yet enraged, presses Seo-yeon for information, sending her spiraling into desperation.
On a fateful day in 1999, Young-sook encounters young Seo-yeon and her father at the house. In a shocking turn, she murders Seo-yeon's father and abducts young Seo-yeon. This cruel twist shifts Seo-yeon's reality yet again—her father disappears, and her home deteriorates to a grotesque state, with the chilling message "pick up the phone!" crudely scrawled on the floor.
In their next interaction, Young-sook intensifies her manipulations, coercing Seo-yeon to divulge her secrets or risk her younger self being harmed. Cornered and running out of options, Seo-yeon learns about an explosion about to occur in 1999 and concocts a cunning plan to lure Young-sook to her doom. However, the explosion transpires just outside as Young-sook exacts revenge on young Seo-yeon, inflicting painful burn scars that resurface in Seo-yeon's adult life.
Left with no alternatives, Seo-yeon bravely infiltrates the local police station in search of critical insights about Young-sook's case. Her investigation leads to the discovery of an officer's notebook from 1999. During their next call, Young-sook attempts to establish a twisted connection, claiming they are two sides of the same coin, revealing the unsettling fact that Seo-yeon unwittingly contributed to her father's demise.
Determined to change the course of their intertwined fates, Seo-yeon, armed with newfound knowledge, informs Young-sook of the junk collector’s real address where she unknowingly hopes to trap Young-sook. However, Young-sook, initially triumphant, spirals further into chaos, continuing her horrifying spree of murders. An even more grotesque reality engulfs Seo-yeon in 2019, revealing a home cluttered with the remnants of Young-sook's victims—and even more haunting, an older Young-sook now residing within.
Seo-yeon soon realizes the notebook's disturbing contents have morphed yet again. It notes her mother's frantic search for the missing Seo-yeon and father, revealing that Eun-ae had also placed a call on Young-sook's phone. Instinctively, Seo-yeon reaches out during this crucial juncture, encouraging her mother to escape.
In 1999, Young-sook, sensing the threat, murders the investigating police officer and relentlessly pursues Eun-ae. Simultaneously, the older Young-sook stalks Seo-yeon in her present. With mounting tension, Seo-yeon pushes her mother to retaliate, leading to a fierce confrontation that escalates—both Eun-ae and Young-sook plummet over the railing in a desperate clash.
As 2019 unfolds in an even more dilapidated state, Young-sook’s foreboding presence dissipates just before delivering a fatal blow to Seo-yeon, granting her a fleeting chance. Reunited, Seo-yeon and her mother find solace, grateful for their survival as they pay homage to her father's grave.
In a gripping mid-credits scene, Young-sook, portrayed as an ominous figure, reaches out to her younger counterpart before the police arrive, ensuring she survives the fall and granting herself the power to reshape her fate, as young Seo-yeon and Eun-ae leave the cemetery, only for Eun-ae to be wiped from existence. The unsettling conclusion reveals a chilling scene in an abandoned torture chamber, where an adult Seo-yeon is left in a terrifying predicament, likely facing an ominous fate by Young-sook.
