After writing a series of articles about a pedophilia scandal, journalist Ji-won receives threatening calls on her cellular and subsequently changes her number. Her close friend Ho-jung, who is infertile, and her husband Chang-hoon invite Ji-won to move to their empty house in Bang Bae that is newly furnished. When Ho-jung's daughter Young-ju answers an anonymous phone call on Ji-won's new cell phone, the girl screams and begins to show a disturbing attraction for her father and jealous rejection towards her mother. Meanwhile, Ji-won receives more mysterious phone calls and sees a teenager playing Beethoven's "Moonlight Sonata" on the piano. After investigating her phone number, Ji-won discovers that the original owner of the number, Jin-hee, had vanished and that the two subsequent owners of the number had died mysteriously in unusual circumstances. Her further investigation about Jin-hee reveals that the teenage schoolgirl was obsessively in love with a man who had broken up with her. Before Jin-hee vanished, Ji-won found out that Jin-hee was in a relationship with Chang-hoon. Ho-jung found out about the affair and confronted Jin-hee. During their confrontation, Jin-hee mocked Ho-jung's infertility problem and told Ho-jung that she was pregnant with Chang-hoon's baby. In a struggle between the two women, Ho-jung accidentally pushed Jin-hee down the stairs and killed her. Ji-won also discovers that Jin-hee's dead body was hidden inside one of the walls at the house she is staying at. However, before Ji-won can go out for help, Ho-Jung arrives and knocks her out. Ho-jung reveals that aside of Jin-hee, Ho-jung is also jealous of Ji-won, who had let her egg be used in in vitro fertilisation so Ho-jung could conceive Young-ju, which technically made Ji-won the true biological mother of Young-ju. It is also revealed that Ho-Jung stages suicide for Chang-hoon, making it seem that he is guilty of Jin-hee's death, so he commits suicide in the bathtub. Ho-jung plans to burn the body of Jin-hee and the unconscious Ji-won with gasoline. However, Jin-hee's spirit awakens and takes revenge on Ho-jung, thus saving Ji-won. The film ends with Ji-won dropping the cursed cell phone into the ocean, which rings after entering the water.
