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.
