Eun-joo was initially a nurse who took care of Su-mi's gravely ill biological mother. However, she soon developed a secret love affair with Su-mi's father and they eventually married each other, which started off the chain of events that has haunted the family ever since.
Knowing about the affair, Su-mi's biological mother soon became depressed and Su-mi and her sister became cold and distant towards their new stepmother. One day, Su-yeon discovered that her biological mother had hung herself in her bedroom closet as a result of her depression.
However, as Su-yeon tried to revive her dead biological mother, the closet accidentally fell on her, causing a loud crash which alerted Eun-joo who was downstairs when the event occurred. Eun-joo initially decided to help Su-yeon but after seeing Su-mi, they had a heated confrontation and after Su-mi told her to stay out of their lives, she became angry and told her that she would regret this moment.
Eun-joo eventually decided to leave Su-yeon to be crushed to death by the weight of the closet after the confrontation. After her Su-yeon's death, she left the family to presumably take a break. During the "break", Su-mi experienced overwhelming amounts of regret, which caused her to slowly become insane and develop multiple personality disorder. Su-mi's multiple personality order worsened, even after being sent to be treated at the mental hospital, which resulted her to act out the roles of her stepmother and her sister.
In the climax of her delusional state where Su-mi had harmed herself to act out the imagined confrontations between her and Eun-joo, Eun-joo returned home to comfort Su-mi and she decided to send her back to the mental hospital. Remorseful, Eun-joo tried to apologise for the pain that she had afflicted towards Su-mi in the beginning but Su-mi refused to accept her apology, leaving Eun-joo frustrated once again as she leaves the mental hospital and returns home.
As she tried to relieve herself alone in the kitchen, Eun-joo heard strange sounds which sounded like footsteps upstairs. Investigating the strange sounds, she eventually found herself in the room where Su-yeon and her biological mother died. The room was unusually cold but as Eun-joo attempted to investigate the room, the door suddenly closed shut and the lights turned off by itself. Noticing the door of the bedroom closet opening by itself, Eun-joo spotted a lace that was stuck between two mattresses that were inside the closet.
Pulling the lace, she accidentally disturbed the vengeful ghost of the biological mother and Eun-joo fell down on the floor and tried to back away as the ghost emerged from the closet. Sticky, white liquid poured out from the closet as the ghost crawled out of the closet and made disturbingly demented noises that sounded like a crying baby. Eun-joo could only look in horror as she saw the ghost come closer towards her and the last time the audience saw her alive was when her screams were heard from outside.
The film does not explicitly state or show her fate but it is presumed that Eun-joo was killed by the ghost as revenge for the pain and suffering that she had inflicted on the girls' biological mother.
- She served as an inspiration for the character Rachel Summers from The Uninvited.