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From the liner notes

While training after hours in her high-school, the aspirant singer Park Young-Eon is mysteriously killed and her body vanishes. Her ghost is invisible and trapped in the school, but her best friend Kang Sun-min, who broadcast in the lunchtime in school, is able to hear her voice. After the suicide of their music teacher, Sun-min, aided by her mate Cho-Ah, finds that another student, Hyo-Jung, died in the same elevator trunk some time ago. Meanwhile Young-Eon recalls details of her life, disclosing why she died.

This film was released overseas as part of the Whispering Corridors series. Don't be fooled by the "Unrated" tag... this film is extremely tame, with minimal gore and no skin whatsoever.