Charlotte is a talented cellist prodigy but had to give up on her training to look after her sick mother. After some years her mother dies, so Charlotte reaches out to her erstwhile mentor, Anton, who runs the prestigious Bachoff music school in Boston where she trained. Anton is temporarily in Shanghai with his two assistants Geoffrey and Theis and his wife Paloma. They are delighted to hear from Charlotte after all this time and ask her to come out and join them to assess some young cellists who are auditioning for a position at Anton's school. One of the judges at this audition is Lizzie, who became Anton's star pupil after Charlotte left. Lizzie is still associated with the school and is now famous. When Lizzie and Charlotte meet they strike up a strong friendship, based on their mutual admiration of each other's talent. They become lovers.After the audition Lizzie has a few days free, so she and Charlotte decide to take a bus trip together before returning from China. They celebrate the evening before and Lizzie wakes up with a dreadful hangover. Charlotte has some ibuprofen which she gives to Lizzie so they manage to get on board the bus, but Lizzie feels steadily worse and worse. Charlotte eventually has to ask the bus driver to stop so that Lizzie can get out and be sick. The driver complies reluctantly but is upset at having his schedule disrupted, so next time Lizzie has to throw up she tries to do so out of the bus window, but fails to open it and vomits all over it instead. Lizzie and Charlotte notice to their horror that Lizzie's vomit contains dozens of insects crawling over the window glass. The bus driver freaks out and chucks them both off, leaving them stranded in the middle of nowhere. Lizzie vomits again, seeing even more bugs, then starts to realize they are crawling all under the skin of her right hand. Charlotte and Lizzie both panic; Lizzie, desperate to get the bugs out of her, grabs a chopper that Charlotte happens to be holding and chops her hand off.Fast forward a couple of weeks and Lizzie turns up at the gate of the Bachoff school minus her hand. A flashback reveals that the bugs weren't real and Charlotte had planned all of this. Instead of giving Lizzie ibuprofen, she gave her a strong hallucinogenic drug that was originally prescribed for Charlotte's mother, and manipulated Lizzie into cutting her hand off. When Lizzie came around, she realized what Charlotte had done and eventually found her way back to Boston. Lizzie tells Anton that Charlotte did all this out of jealousy and begs to be allowed back into the school, but Anton tells her she is no use now she can't play the cello any more and sends her away.Filled with anger, Lizzie tracks Charlotte down to her home in Minneapolis, knocks her out with a taser, then ties her up and takes her back to the Bachoff in the trunk of her car. Anton is pleased to have Charlotte returned to him and questions her to find out why she did what she did to Lizzie. It turns out (through a series of flashbacks) that Charlotte was systematically abused while she was at the school, punished under a brutal regime of rape and torture whenever she failed to achieve musical 'perfection', suffering severe psychological damage as a result and having to spend time in a mental institution after she left the school and before caring for her dying mother. Charlotte's only purpose was to save Lizzie from the same fate, after recognizing a tattoo of an eighth-note on Lizzie's back as a symbol carried by each girl indoctrinated into the sex cult run by Anton, Geoffrey and Theis. Paloma was fully complicit in all of this.Anton is enraged and tells Charlotte she has one chance to go free: to play a difficult musical piece to perfection in front of his new pupil from Shanghai, whom he will 'punish' if she makes a mistake. Charlotte tries her best, but makes a single error which Anton reacts to by sending his new pupil to bed and telling Lizzie, Geoffrey and Theis to do whatever they liked to Charlotte and come get him when she stops struggling. Geoffrey and Theis advance on Charlotte but Lizzie says she wants first turn, to rape Charlotte with the stump of her arm because of what Charlotte did. Geoffrey and Theis wait eagerly, but then fall down dead because Lizzie had poisoned their drinks while they were watching Charlotte play. It turns out that Charlotte had already explained to Lizzie that she had wanted to save her from Anton; Lizzie believed her and they planned all of this together.Anton, meanwhile, is waiting in his room when he gets a knock on the door. It is Paloma, who is fumbling and hesitant because she has a knife stuck in her back. As Paloma collapses, Lizzie and Charlotte come in behind her, bearing kitchen knives. Anton struggles, tries to fight them off, manages to get Charlotte's knife off her and causes severe damage to her left arm, but is eventually overpowered and struck repeatedly by Lizzie.The film ends with Charlotte and Lizzie doing an impromptu concert on the cello. Lizzie is doing the fingering with her left hand and Charlotte is doing the bowing with her right. Their audience of one is Anton, who sits in a chair with his arms and legs amputated and his eyes and mouth sewn shut, listening to the girls play.