context: The viewer first sees a young woman playing the Ave Maria on a cello, the scene shifts to show another woman bleeding on an operating table.Another scene shift shows the main character, Hong Mi-ju (Seong Hyeon-ah) watching students playing their cellos while their professor grades them. The professor, being friendly with Mi-ju, tried to coax her into going to a welcome home concert for the little sister of Kim Tae-yeon (Tae-yeon, as we find out, is deceased). Mi-ju appears nervous and politely declines both the concert and a job offer to become more than just an associate teacher. In the teacher's lounge, Mi-ju is confronted by an angry student who tells Mi-ju that all because of Mi-ju, all of her work for music is for nothing. The student is smug and cocky, promising revenge and asks Mi-ju "Are you happy now? You should be!". Mi-ju leaves, shaken, and starts to drive home. On her way back home, she nearly avoids getting into an accident with a truck, and discovers she has luckily avoided death. When she gets home, Mi-ju receives a message on her cell phone: "Are you happy? ...You should be."Startled, Mi-ju looks around the house while turning on lights, but they snap off when she reaches the attic. There, she sees her elder, autistic daughter, Yoon-jin (Choi Ji-eun), sitting in a chair, facing the window. The lights come on, revealing her husband Jun-ki (Jeong Ho-bin), her sister-in-law Kyeong-ran (Wang Bit-na), and her younger daughter Yoon-hye. Everyone but Yoon-jin, who appears mute and emotionless, sings "Happy Birthday" to Mi-ju. Mi-ju opens her presents, which include a music album from Kyeong-ran, "Rainy Day". Yoon-hye shows her mother how she has taught her older sister to count to five with her fingers, which seems to fascinate Yoon-jin. Later on, as they bathe, Mi-ju tells Yoon-jin how happy she is, how much she loves Yoon-jin. Mi-ju seems to share a special affection for Yoon-jin, as she blames herself for Yoon-jin's autistic condition.The next day after a doctor's visit, they pass a music store. Yoon-jin stops to stare at a cello, and Mi-ju buys it for her. When they get home, their dog, Sunny, barks uncontrollably at them, but stops once Yoon-jin and the cello pass by. Mi-ju meets the new, silent housekeeper, Ji-sook, and begins to teach Yoon-jin how to play the cello. The next morning, however, Sunny the dog is found dead.More strange things happen; Yoon-hye plays in Kyeong-ran's room until the latter's fiance, Hyeon-woo, calls. Having been shooed out, Yoon-hye goes to her sister's room to ask to try the cello, but the normally calm and emotionless Yoon-jin surprisingly bites her little sister.The same night, Kyeong-ran has a breakdown, as her fiance has seemingly broken up with her. There seems to be no calming her down, and eventually the family has to just leave Kyeong-ran alone for the time being. In Yoon-jin's room, Mi-ju watches the sleeping Yoon-jin, but her daughter's sleeping face suddenly becomes ghastly, and eerie voices can be heard.In Kyeon-ran's room, a crack appears on a picture of Kyeong-ran and her fiance, and through the wall, a ghost (who oddly enough looks like the disgruntled student who confronted Mi-ju about the poor grade she had received) emerges and throws her through the glass balcony door. The rest of the family does not seem to hear a thing, calmly trying to knock on Kyeong-ran's door again to get her to come out for something to eat. However, Yoon-jin gets out of bed and pulls back the curtain, and sees Kyeong-ran strangled and dangling at her window.Sensing his wife's inner turmoil, Jun-ki asks what's going on, and presents her with her old college yearbook, asking why Kim Tae-yeon's pictures are cut out. Mi-ju tells her husband the reason why she quit playing the cello is because of her former friend, Kim Tae-yeon. They could have been great friends, Tae-yeon had told Mi-ju, but Tae-yeon was tired of always being in Mi-ju's shadow. The flash back shows a plain girl, Kim Tae-yeon, struggling to play as good as her friend, and struggling to pretend to be happy for her friend as Mi-ju rises far and above her friend. Mi-ju tells her husband that she doesn't think Tae-yeon was trying to kill her out of jelousy, but the night Mi-ju was chosen above Tae-yeon, there was a car accident and Tae-yeon was killed while Mi-ju was merely injured.Despite the shock of her sister-in-law's apparent suicide, Mi-ju attends the cello concert she had previously declined. However, the strange circumstances don't stop there, and with a blink of an eye, Mi-ju finds herself alone in the room. She sees the same ghost who emerged through Kyeong-ran's wall on stage playing the cello, and hears Yoon-hye's voice whispering, "One, two, three, four, five." But then, it is over and the other people and the cello player reappear, but Mi-ju leaves, terrified, hearing the voices from the night before.At home, Yoon-hye begs again to play the cello, but becomes frightened when the cello starts to appear and disappear without anyone touching it. She tries to leave Yoon-jin's room, but the door is locked. They are suddenly transported to the balcony, with Yoon-hye hanging from the side. Mi-ju returns to see Yoon-jin pulling Yoon-hye's fingers back one by one as the voices repeat, "One, two, three, four, five." Yoon-hye falls to her death, and it begins to rain. Mi-ju gathers her now dead little daughter, and places her body in the basement, unable to cope with what has just happened, and trying desperately to protect her older daughter, Yoon-jin. When Mi-ju's husband, Jun-ki, returns home from work, he asks where Yoon-hye is, but Mi-ju replies she sent her daughter to camp. Wanting to talk to his little girl, Jun-ki calls her cell phone, but gets no answer. However, he starts to hear the ringing of the phone in the basement where Yoon-hye's body is...Confronting his wife, he demands to know what's going on, accusing Mi-ju of killing their little girl. As Mi-ju explains it was an accident, starting to get hysterical. In the struggle, Mi-ju pushes her husband back, only to find he has been stabbed by a sharp pipe and is dead. Mi-ju turns to face the stairway slowly and sees the ghost who looks like the disgruntled student and whispers, "...Kim Tae-yeon..."Through the ghost of Kim Tae-yeon's eyes, we see a flashback and the truer events. Kim Tae-yeon, who looks exactly like the student from the beginning of the movie; pretty, talented, and confident (indeed, the audience is led to believe there really wasn't a student to begin with, but rather Mi-ju hallucinating events), is the more talented cello player. It is Kim Tae-yeon, not Mi-ju, who was the more talented, sweet, and "better" student. After the final humiliation of Kim Tae-yeon being chosen of, Mi-ju swerves while driving them home and crashes the car by a steep incline off the road. Tae-yeon is thrown from the car and is barely holding on from falling off the side of the cliff. Mi-ju grabs her friend's hand but eventually lets Kim Tae-yeon fall to her death.Brought back to the present, Mi-ju tries to stab Tae-yeon's ghost with a knife to stop her from going to Yoon-jin, begging "Not my Yoon-jin, not my Yoon-jin!". She then sees that she has stabbed the housekeeper. Believing the cello to hold the power to the ghost of Kim Tae-yeon, Mi-ju grabs a golf club and rushes to Yoon-jin's room. Mi-ju grabs the cello, throws it out against the wall, and smashes it as Yoon-jin screams in her room. When the noise stops, Mi-ju goes in. The room is empty except for the cello, which is unharmed. She looks back into the hallway to see the beaten and bloody body of her daughter. As Mi-ju kneels by the weltering Yoon-jin, she feels Tae-yeon forcing her hand to stab Yoon-jin's beaten body. Mi-ju resists and stabs herself in the chest.Mi-ju wakes up in the hospital to find that her earlier car accident was not imaginary, and that the previous events have been part of her comathe voices she heard were actually those of Jun-ki and Yoon-hye, whispering for her to wake up. Mi-ju finds her family members are all safe and sound, around her and hugging her tightly. Outside the hospital room, Jun-ki asks the doctor why his wife thought they were all dead. The doctor replies it was probably part of the comaMi-ju's family is all that is important to Mi-ju now, and oftentimes anxiety will pertain to what is most important to a person.When Mi-ju returns home, she receives the same message again: "Are you happy? ...You should be." She walks through the house turning on lights, which go out when she reaches the attic. Again, she finds her family there, in the same way as she did in the beginning of the movie. They sing "Happy Birthday" to her, and Kyeong-ran gives Mi-ju the same album. Inside, she finds a scribbled inscription: "This is only the beginning," before the ghostly hands of Kim Tae-yeon reach through Mi-ju's hair and slowly grasp her old friend's face.
question: What does Yoon-jin stare at in the music store?
answer: Cello