After writing a series of articles about pedophilia, the journalist Ji-won receives threatening calls on her cellular and she changes her number. Her close friend Ho-jung and her husband Chang-hoon invite Ji-won to move to their house in Bang Bae that is empty and closed. When the young daughter of her friends Young-Su answers a phone call in her mobile phone, the girl screams and changes her behavior, feeling a great attraction for her father and rejecting her mother.
Meanwhile Ji-won receives weird phone calls and sees and listens to 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 the two next owners of the number have mysteriously died in unusual circumstances.
Her further investigation about Jin-hee discloses that the teenager was absolutely disturbed with her obsessive love for a man that had broken the relationship with her, and later she unravels dark and tragic secrets about the fate of Jin-hee.