The real schoolgirl Sadako is not the evil "Sadako" who appears in the (chronologically) later films; rather, the real Sadako is haunted by her evil spirit twin, and has to fight to prevent her breaking through and taking control. The members of the drama school blame the real Sadako for the deaths of the students and beat her to death. They travel to her father's house, taking the corpse with him, believing they'll be able to destroy the evil spirit if they go there.
Instead, the evil spirit brings Sadako back to life, and - her trust
in humanity having been betrayed - the real Sadako abandons her
efforts, allowing the evil Sadako to break through and psychically
kill all of the drama school members. Seeing that the evil spirit is
now out of control, Sadako's father (who, it is strongly suggested, is
not really her father at all) seals her in the well.
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Read more about this film at The Internet Movie Database.