Trama

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.

Seo-yeon, 28, takes the train to the South Korean countryside to visit her hospitalized mom. Having lost her smartphone on the train, she finds a phone in her mom's house. She finds a hidden basement and Young-sook's 20 year old/Nov.1999 diary when she, too, was 28. Her shaman stepmom believes her stepdaughter will become a serial killer if let out and is very cruel to her. The two 28 y.o. women start talking together on the same phone in the same house but separated by 20 years. Can Seo-yeon's dad, who died then, be saved? Can history be changed? Should it?—Scott Filtenborg

In 2019, Seo-yeon returns to her childhood home in the countryside after visiting her estranged mother, who has a brain tumor, in the hospital. While walking, Sung-ho, who owns a strawberry plantation, gives a ride to her in his truck. Seo-yeon realizes that she lost her cellphone in the train and connects a cordless telephone she finds in the house. Soon she receives several phone calls from a distressed woman and they start talking to each other. She learns that the other woman is Young-sook and they both are twenty-eight years old and have problems with their mothers. Further, Young-sook lives in the same house in 1999 and her stepmother is a shaman, who is cruel to her, keeping her locked up in the house and punishing her for mistakes. When Seo-yeon tells Young-sook that she blames her mother for the fire that killed her beloved father in 1999, Young-sook saves her father and the life of Seo-yeon changes for happiness with her family together. When Seo-yeon learns that the mother of Young-sook killed her during an exorcism, she informs Young-sook and she kills her mother instead. Soon Sung-ho and her father disappear and Seo-yeon learns that Young-sook is a psychopath and sociopath and has become a serial-killer. But what can she do to stop Young-sook living twenty years in the future.—Claudio Carvalho, Rio de Janeiro, Brazil

In 2019, 28-year-old Kim Seo-yeon loses her cellphone while traveling to her old childhood home in a rural area, where she has come to visit her sick, estranged mother in the hospital. Arriving at the rundown house, she manages to reach the people who have found her phone, but they attempt to swindle her for money in exchange for it. She searches the house and finds a decades-old cordless phone to use instead. The first call she receives on it is from a distressed woman who says she's being tortured by her mother. After investigating the house, Seo-yeon discovers a hidden basement, where she finds leftover material from the previous owners. She figures out that the woman on the phone is Oh Young-sook, who lived in the same house but in 1999.

When they talk again, Seo-yeon shares her revelation that the two are able to communicate across time through the phone, and they exchange information about their lives. Young-sook is orphaned and lives with her adoptive mother, a shaman who mistreats her, while Seo-yeon lost her father in a fire that she blames her mother Eun-ae for, who had once left a stove burning; Seo-yeon still has scars from the ensuing fire. At that moment, the shaman welcomes an 8-year-old Seo-yeon with her parents at her house, who are in the process of buying it. Young-sook uses the phone to allow Seo-yeon to overhear the conversation, and hear her father's voice one more time.

Young-sook, acting on information from Seo-yeon, sneaks out of the house in 1999 to prevent the fire that will kill Seo-yeon's father. She is successful, and Seo-yeon's 2019 reality suddenly changes: her parents are both alive and healthy, their house looks lavish, and her scars have disappeared. Young-sook, however, is severely punished by her mother for leaving, and becomes resentful that Seo-yeon's life is improved while hers remains the same.

Seo-yeon starts searching the internet for more information that may help, and learns that Young-sook was killed by her mother during an exorcism, who was arrested for it. During the next phone call, Seo-yeon warns Young-sook about this. When the shaman starts planing for the exorcism by claiming to see a dark future in Young-sook, she saves herself and kills her mother instead.

In 2019, Seo-yeon enjoys a dinner with her parents and with Sung-ho, the local strawberry farmer. However, in 1999, a now freed Young-sook starts giving into her killer instinct. She is friends with the younger Sung-ho, who has also delivered strawberries to her, but when he accidentally discovers the shaman's chopped-up remains inside her refrigerator, she kills him. In 2019, Seo-yeon again experiences a change in her reality, where Sung-ho suddenly disappears with neither one of her parents having any recollection of him, and his strawberry farm no longer exists. Seo-yeon does another internet search, learning that Young-sook was eventually arrested for being a serial killer. During the next phone call, Seo-yeon confronts Young-sook with this, but inadvertently reveals to her that she'll be in jail for life. Young-sook demands to know how the police will find out about this, but Seo-yeon refuses to tell.

In 1999, Young-sook is suddenly visited by 8-year-old Seo-yeon and her father again, who have come to the house to close their purchase of it, after the shaman did not turn up at the realtor's office that afternoon. Young-sook then kills Seo-yeon's father and takes young Seo-yeon captive. In 2019, Seo-yeon's reality changes again: her father disappears, and she finds her house in even worse condition; the words "pick up the phone!" are chalked on the floor. Young-sook calls Seo-yeon again, and tells her to reveal how she'll be arrested, or she will harm Seo-yeon's younger self.

Although Seo-yeon cannot change the past, she goes onto the internet and learns that there was a gas explosion that took place shortly after their last conversation, which destroyed a building in the neighborhood back in 1999. She tells Young-sook that the knife that she used to kill her stepmother with and then disposed of was found by a junk collector in 1999, who took it to the police; she feeds Young-sook false info by giving her the collector's supposed address, which in reality is the address of the building that is going to explode, hoping to lure Young-sook to her death that way. However, due to a coincidence, Young-sook is just outside when the explosion happens. Out of revenge, Young-sook pours hot water onto the young Seo-yeon, causing agonizing burn scars to appear on the older Seo-yeon in 2019. Young-sook then threatens to kill Seo-yeon's mother Eun-ae next, unless she starts helping for real.

Seo-yeon has no choice but to break into the local police station to look for information about Young-sook's case. She finds an old notebook used in 1999 by the officer handling the case. During their next call, Young-sook taunts her by saying they are the same, because Seo-yeon herself was once responsible for the fire that originally killed her father: Eun-ae had, in fact, turned off the gas when she left, but young Seo-yeon had put it back on, and then blamed her mother for it. Seo-yeon uses the police notes to provide Young-sook with the junk collector's real address, who finds and kills him in 1999, and destroys the knife. With no evidence linking her to the murder, she is not arrested and continues her killing spree. Seo-yeon's 2019 reality changes once again: her house is even more dilapidated, and filled with refrigerators, all containing body parts of her many victims. Even worse, an older Young-sook still owns the house, and lives there now.

Seo-yeon realizes that the content of the notebook has also changed: it now contains notes from the policeman when he came to the house together with Eun-ae, looking for the missing young Seo-yeon and her father in 1999. Young-sook allowed them to search the house, but they left when nothing was found. The note also mentions that Eun-ae made a call from Young-sook's cordless phone while in the house, so Seo-yeon calls around that approximate time, speaks to her mother and warns her to leave.

In 1999, Young-sook learns what Seo-yeon is trying to do, so she kills the police officer and chases Eun-ae around. Meanwhile, in 2019, the older Young-sook reveals herself and similarly chases old Seo-yeon through the house. Eun-ae uses the phone again in 1999, and Seo-yeon picks up in 2019, encouraging her mother to fight back. Eun-ae does so, fighting off Young-Sook with significant injury, and seemingly sacrificing herself by pushing her over the first-floor railing, with both tumbling over and landing on the floor below. 2019 changes again, with the house becoming derelict, and old Young-sook disappearing just before she can deliver a killer blow to Seo-yeon. She leaves the house and is emotionally reunited with her mother, who survived the 1999 events, albeit with scars. They visit her father's grave and then depart.

In a mid-credits scene, we see a flashback to the older 2019 Young-sook placing a call before her final confrontation with Seo-yeon: she uses the phone to call and warn her younger 1999 counterpart just before Eun-ae and the police officer arrive. This allows 1999 Young-sook to survive her fall from the railing and change her own history: as Seo-yeon and Eun-ae leave the cemetery, Eun-ae gets erased from reality. 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 adult Seo-yeon, whom will most likely meet her demise by Young-sook.