Hong Mi-ju (Seong Hyeon-ah) watches students playing their cellos. As a locker, a girl tells her that a poor grade from Mi-ju has kept her from music college. The student grabs Mi-ju, who cries out and pulls back her wrist, on which are three large but not new scars. On the drive home, Mi-ju turns on the radio to hear a cello. Her scars begin to writhe. She hallucinates about being in a car accident. When she gets home, she receives a message on her cell phone: "Are you happy? You should be."
Mi-ju looks around the house while turning on lights, but they snap off when she reaches the attic. There, she sees her elder daughter, Yoon-jin (Choi Ji-eun). The lights come on, revealing her husband Jun-ki (Jeong Ho-bin), her sister-in-law Kyeong-ran (Wang Bit-na), Yoon-jin, 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. As they bathe, Mi-ju tells Yoon-jin how happy she is. As they pass a music store the next day, Yoon-jin stops to stare at a cello. Mi-ju buys it for her. When they get home, their dog, Sunny, barks uncontrollably at them. Mi-ju meets the new housekeeper, Ji-sook, and teaches Yoon-jin to play the cello. In the morning, Mi-ju finds Sunny dead.
Yoon-hye plays in Kyeong-ran's room until the latter's fiancee, Hyeon-woo, calls. Yoon-hye goes to her sister's room to ask to try the cello, but Yoon-jin bites her. Kyeong-ran shuts herself in her room and scream for the others to leave her alone.
As Mi-ju watches, the sleeping Yoon-jin's face becomes ghastly, and eerie voices can be heard. A crack appears on a picture on Kyeong-ran's wall, through which a ghost emerges and throws her through the glass balcony door. Yoon-jin gets out of bed and sees Kyeong-ran strangled and dangling at her window.
Mi-ju attends a cello concert. When the cello player leaves the stage, Mi-ju finds herself alone in the room. She sees a ghost on stage playing the cello, and hears Yoon-hye saying, "One, two, three, four, five." The other people and the cello player reappear, but Mi-ju leaves, terrified. She hears the voices from the night before.
At home, Yoon-hye is scared by the cello and 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. Jun-ki finds Yoon-hye's body and assumes that Mi-ju killed her. He dies in the confrontation. Mi-ju then sees Kim Tae-yeon's (Park Da-an) ghost.
In a flashback, a younger Tae-yeon and Mi-ju are talking about the song "Rainy Melody". Later, Mi-ju is driving with Tae-yeon in the passenger's seat. Tae-yeon asks if Mi-ju is happier when they are together, but Mi-ju tells her to be quiet. They get into a car accident. Mi-ju regains consciousness to see Tae-yeon dangling from the open door of the car, which is hanging over a cliff. Mi-ju takes her hand, but remembering Tae-yeon's superior talent and recognition, lets go. Tae-yeon holds onto Mi-ju's wrist, leaving deep scratches, but eventually falls.
In the present, Mi-ju stabs Tae-yeon with a knife to stop her from going to Yoon-jin. She then sees that she has stabbed the housekeeper. 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. She looks back into the hallway to see the beaten body of her daughter. As Mi-ju kneels by the weltering Yoon-jin, she feels Tae-yeon forcing her hand to stab Yoon-jin. 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 coma--the voices she heard were actually those of Jun-ki and Yoon-hye. When she returns home, she receives again a message: "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. They sing "Happy Birthday", and Kyeong-ran gives Mi-ju the same album. Inside, she finds a scribbled inscription: "This is only the beginning."