Two people live in different times. Seo-Yeon lives in the present and Young-Sook lives in the past. One phone call connects the two, and their lives are changed irrevocably.
In 2019, 28-year-old Kim Seo-yeon loses her cellphone while traveling to visit her sick, estranged mother in a rural area. Arriving at her rundown childhood home, she finds a decades-old cordless phone, and through it receives calls from a distressed woman who says she's being tortured by her mother. After investigating the house, Seo-yeon figures out that the woman on the phone, Oh Young-sook, is living in the same house but in 1999. The two are able to communicate across time through the phone, and exchange information about their lives. Young-sook is orphaned and lives with her adoptive mother, who is a shaman, while Seo-yeon lost her father in a fire that she blames her mother, Eun-ae, for.
Young-sook, acting on information from Seo-yeon, sneaks out of the house to prevent the fire that killed Seo-yeon's father. She is successful, and Seo-yeon's reality changes: her parents are both alive and healthy, and their house lavish. Young-sook, however, is punished by her mother, and becomes resentful that Seo-yeon's life is improved while hers is the same.
Seo-yeon searches the internet and learns that Young-sook was killed by her mother during an exorcism. During the next phone call Seo-yeon warns Young-sook, who saves herself and kills her mother instead. Now freed, Young-sook becomes a serial killer. Seo-yeon realizes what's happened when Young-sook's victims disappear in the present day. During a phone call, Seo-yeon confronts Young-sook, but inadvertently reveals to her that she'll be arrested.
In 1999, Young-sook is visited by an 8-year-old Seo-yeon and her father, who have come to the house to close their purchase of it. Young-sook kills Seo-yeon's father and takes young Seo-yeon captive. In 2019, Seo-yeon's reality changes again: her father is dead and the house in even worse condition. Young-sook calls Seo-yeon and tells her to find out how she'll be arrested. At first Seo-yeon feeds Young-sook false info, but when Young-sook threatens to kill Eun-ae next, Seo-yeon breaks into the local police station for the notebook used in 1999. Young-sook taunts Seo-yeon that they're the same, because Seo-yeon caused the fire that originally killed her father, and lied about Eun-ae being responsible.
Seo-yeon gives Young-sook the correct info, and her reality changes again: the house is now owned by an older Young-sook, who has continued unstopped as a serial killer. The content of the notebook changes as well, with a note that Eun-ae came to the house with a police officer and made a call on the cordless phone. Seo-yeon waits in the house for the call, and uses it to warn Eun-ae.
In 1999, Young-sook kills the police officer and chases Eun-ae. In 2019, older Young-sook reveals herself and similarly chases Seo-yeon. Eun-ae uses the phone again, and Seo-yeon picks up and encourages her to fight. Eun-ae seemingly sacrifices herself to kill Young-sook, and 2019 changes, with the house becoming derelict and old Young-sook disappearing. Seo-yeon leaves the house and is emotionally reunited with Eun-ae, who is alive and well, albeit with scars.
In a mid-credits scene, older Young-sook uses the phone to call her younger counterpart and warn her about Eun-ae and the police officer, allowing her to change her own history, and erasing Eun-ae from Seo-yeon's side. At the end, the scene cuts to the torture room where a person covered in white cloth is tied to a chair, screaming for help. The cloth is removed, revealing a frightened Seo-yeon, whom will most likely meet her demise by Young-sook.