From the get-go, it’s clear there’s something sinister going on. At first, it seems like it might be ghosts. Su-mi has constant nightmares about the ghost of her mother, and when her aunt comes for dinner one night she sees the ghost of a dead girl as she suffers a seizure.
However, it becomes clear that there’s something more human to all of this, with Eun-joon abusing the girls while their dad is completely oblivious to it. Su-yeon gets the worst of it, even getting locked in a wardrobe closet by her stepmother for no reason. When Su-mi finally snaps and confronts her dad about the abuse towards her sister, he reveals Su-yeon is actually dead — despite her standing next to Su-mi. This is the first big twist of the film. All along, Su-mi has been imagining her dead sister.
Ghost or not, there’s still the matter of Eun-joo lashing out at the girls — well, girl. The stepmother becomes more erratic, plagued by what she thinks are even more ghosts. After Su-mi sees what she thinks is Eun-joon dragging her sister in a bloody sack, they get into a fight. Eun-joo nearly beats her stepdaughter to death and leaves her unconscious. That’s when the second big twist in the movie (there are many!) arrives, as Eun-joo sees her husband arrive — with Eun-joo. This Eun-joo looks healthier than the one we know, and is wearing a suit. She talks to Eun-joo, but refers to her as Su-mi.
That’s right. Turns out, Su-mi was not only seeing her dead sister, but she had a split personality and thought she was her own stepmother. The only ghosts were the ones on Su-mi’s head.