In 2019, 28-year-old Kim Seo-yeon finds herself in a challenging situation after losing her cellphone during a trip to her childhood home, where she plans to visit her estranged mother in the hospital. Arriving at the dilapidated house, Seo-yeon manages to contact the individuals who found her phone, but they attempt to exploit her by demanding money for its return. Turning to a decades-old cordless phone she discovers, she receives an unsettling first call from a distressed woman claiming she's being tortured by her mother.
Through her investigation of the house, Seo-yeon uncovers a hidden basement filled with remnants left by previous occupants. She soon realizes that the voice on the other end of the line belongs to Oh Young-sook, a girl who lived in the same house in 1999. The two engage in a remarkable exchange, discovering that they can communicate across time through the phone. Young-sook reveals her grim reality: she is an orphan mistreated by her adoptive mother, a shaman, while Seo-yeon shares her own traumatic past, marked by the tragic loss of her father in a fire, a tragedy she blames on her mother, Eun-ae, for leaving the stove on.
As their conversations unfold, Young-sook cleverly allows Seo-yeon to overhear familiar voices from her past, including that of her father. Prompted by Seo-yeon's information, Young-sook decides to act in 1999, sneaking out to prevent the fire that would take Seo-yeon's father's life. Her success alters Seo-yeon’s present: her parents are alive and well, and the house has transformed into a luxurious abode, with Seo-yeon's scars mysteriously healed. However, this change comes at a steep cost for Young-sook, as she faces severe punishment from her adoptive mother, who is angered by Young-sook's defiance.
Determined to help, Seo-yeon turns to the internet for more information, uncovering the harrowing fate of Young-sook; she was ultimately killed during an exorcism gone wrong. Concerned, Seo-yeon attempts to warn Young-sook during their next call. In a desperate bid for power, Young-sook takes drastic measures to escape the path laid out for her, committing murder in the name of survival.
In the present, Seo-yeon enjoys a family dinner with her parents alongside Sung-ho, the local strawberry farmer, while Young-sook in 1999 spirals deeper into danger, eliminating anyone who threatens her secret. A chilling revelation washes over Seo-yeon; as she searches for answers, she learns that Young-sook has been arrested as a serial killer, igniting a new intensity in their conversations. The stakes rise as Young-sook's actions lead to horrific consequences for Seo-yeon, resulting in an even more unstable reality for her.
Things come to a head when 8-year-old Seo-yeon and her father inadvertently enter Young-sook's dangerous life as they finalize their house purchase. In a shocking turn of events, Young-sook captures Seo-yeon's father, causing Seo-yeon’s present to spiral into chaos as memories of her father begin to fade. The haunting message “pick up the phone!” scrawled on the floor signals to Seo-yeon that time is of the essence.
In a gamble to regain control, Seo-yeon discovers vital information about a gas explosion linked to Young-sook's past, devising a plan to lure her into a trap. As the clock ticks, the tension escalates further, leading to a series of violent confrontations that unravel their intertwining fates. Ultimately, the battle for survival culminates as Seo-yeon fights to protect her family, leading to a heartbreaking showdown where both past and present collide.
In a gripping mid-credits scene, a flashback reveals Young-sook's earlier intervention, where she foils her own demise by warning herself in 1999. This act alters the timeline irrevocably, culminating in an eerie conclusion as Seo-yeon's fate hangs in the balance. As the camera fades, the chilling sounds of screams echo, hinting at the formidable threat Young-sook continues to pose, ensnaring Seo-yeon in a nightmarish cycle from which escape seems impossible.
