Plot

In the opening sequence we see a young, frightened woman run into an elevator. Her cellphone rings, the elevator stops, and the lights flicker. She screams to be left alone, and claws at the walls.

We cut from that to a young journalist, Ji-won who has exposed an underage sex scandal. Her editor Ho-jeong warns her to keep a low profile until the men implicated can be tried. In this vein, the journalist purchases a cellphone with an unregistered phone number, and accepts an offer by the editor and her husband Chang-hoon to move into their unused new house in order to keep a low profile for a few days. Ji-won begins to receive mysterious and disturbing phone calls. One of these calls is answered by the editor's daughter, Yeong-ju, who begins to act strangely afterwards and seems to develop an Electra complex.

The journalist examines the call records and by chance discovers that those who previously had her number have all died under mysterious circumstances (the girl in the elevator from the opening scene was one of these previous owners). Throughout her investigations a female ghost keeps reappearing and making her presence known in other ways such as playing the piano, in particular the song Moonlight Sonata by Beethoven. We also learn that Chang-hoon and Ho-jeong only managed to have their daughter through artificial insemination, the eggs having been donated by Ji-won. The mysterious phonecalls to Ji-won continue; she can decipher that they are from a female voice who is screeching about a man with whom she is very angry, but she does not know who or why.

Ji-won learns that the first number holder was a high school girl Jin-hie who went mysteriously missing. By talking to her friends, Ji-won finds out that this girl was madly in love with a man, whom she refused to name to her friends. All the while, Yeong-ju becomes more aggressive toward her mother and makes romantic advances toward her father.

Ji-won discovers that was Jin-hie was having an affair with Chang-hoon (whose favourite piece of music is Moonlight Sonata) and she was four weeks pregnant with his child. We see, in a flashback, the confrontation between the wife and the mistress during which Jin-hie mocks Ho-jeong by mentioning her infertility and taunts her with all the places she and the editor's husband made love. Jin-hie attempts to choke Ho-jeong, but Ho-jeong manages to throw her down the stairs and finishes her off with a heavy sculpture. Having previously offered to decorate the new holiday home while her husband is away, Ho-jeong takes the opportunity to hide Jin-hie's body in the walls of the house with her mobile phone still in her hand.

Back in the present and in the same location, Ji-won is putting the pieces of the mystery together and conludes that Chang-hoon killed Jin-hie. However, moments later he is seen lying dead in the hallway and we realise Ho-jeong is responsible for all the deaths. Ho-jeong douses the house in an accelerant and is preparing to torch Ji-won and Jin-hie's corpse, when Jin-hie's body starts to move within the walls. She reanimates herself and advances upon a petrified Ho-jeong. In the morning Ji-won awakens to see Ho-jeong dead; she slips her bonds and escapes as the camera shows Jin-hie's corpse in the wall once more.

The final scene shows Ji-won throwing the troublesome mobile phone off a cliff. As the phone enters the water it begins to ring as the credits roll.