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This is from Wikipedia
The film's fractured narrative and severely unreliable narrative can make it difficult to piece together, and several key plot elements go unexplained. The narrator is Su-mi who is later revealed to be mentally ill and therefore dubious in her account of the story.
Though we are led to believe in the beginning that the main action of the movie is a flashback being told in the interview to the doctor, it in fact proceeds mainly chronologically; the "day" the doctor wants Su-mi to talk about is the day her mother and sister died.
The narrative is further confused because Su-mi hallucinates the presence of the stepmother and sometimes imagines that she "is" the stepmother. Thus, when it appears the father gives pills to Eun-joo, he is actually giving them to Su-mi; the stepmother's bizarre dinner behavior is actually Su-mi trying to act like her stepmother. This is further proven when Su-mi, Eun-joo, and Su-yeon all begin their menstrual periods on the same day. Su-mi's guilt over Su-yeon's death leads her to make instances where she must protect her sister. Su-mi kills the bird to provide her imagined stepmother with a reason to kill her sister, thus allowing her and Eun-joo to come to the film's climax of conflict between the two. It is also debatable as to whether Su-mi is really hallucinating the presence of her sister, or if she is interacting with Su-yeon's ghost (Su-yeon's actual ghost does seem to make an appearance at least once or twice in the film, although these appearances could also be explained as being hallucinations). The tent that the brother and his wife drive by in the road is nothing more than a tent set up by harvesters and is a common sight on Korean roads in the country. There are many unexplained elements in the movie, but fanatics of the film note that this is true with many Asian films and is one of the main reasons for its popularity; the viewer can make his/her own decisions about what exactly happened. There is a fear among fanatics that if there is an American remake, a concrete explanation will be presented and therefore detract from the film's ambiguity.
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