Charlotte, a gifted cellist prodigy, was forced to set aside her musical training to care for her ailing mother. Following her mother's passing, she reaches out to her former mentor, Anton, who now runs the esteemed Bachoff music school in Boston. He is currently in Shanghai with his wife, Paloma, and his assistants, Geoffrey and Theis. Ecstatic to reconnect with Charlotte, they invite her to join them in evaluating young cellists auditioning for a position at the school.
Among the judges is Lizzie, who became Anton's standout pupil after Charlotte's departure. Their initial encounter ignites a deep friendship rooted in mutual respect for their talents, blossoming into a romantic relationship. After the auditions, Lizzie has a few days off, prompting her and Charlotte to embark on a bus trip across China. They celebrate the night before departure, but Lizzie wakes up the next day dreading the consequences of her hangover. Charlotte offers her ibuprofen to alleviate Lizzie's discomfort, and they manage to board the bus together. Nonetheless, Lizzie's condition deteriorates, leading to an urgent request to the driver for a stop, resulting in a chaotic moment when Lizzie vomits—only to discover a horrifying swarm of insects within it.
The driver, in a fit of panic, ejects them both into the middle of nowhere. Stricken by the sight of the crawling insects, Lizzie soon realizes the shocking truth: the bugs are under her skin. In a desperate attempt to rid herself of this torment, she impulsively seizes a chopper from Charlotte and amputates her own hand.
Fast forward to weeks later; Lizzie arrives at the gates of Bachoff, but her past has left her scarred physically and emotionally. In a flashback, it's revealed that the bugs were mere figments, the result of a powerful hallucinogenic drug Charlotte had given her in place of painkillers. Heartbroken and betrayed, Lizzie begs Anton to be readmitted, but he coldly informs her that she is no longer of any use without her ability to play.
Fueled by rage, Lizzie seeks revenge on Charlotte in Minneapolis, thanks to a stun gun that incapacitates her. She brings Charlotte back to Bachoff, where Anton questions her intentions. Through a barrage of flashbacks, Charlotte’s harrowing past is unveiled: she endured a brutal regime of abuse at the school, meant to foster 'perfection.' Recognizing the same tattoo on Lizzie, Charlotte's only mission had been to protect her from a similar fate.
Anton, furious, gives Charlotte one final ultimatum: play flawlessly to save herself. Despite her best efforts, a single mistake sends Anton's new student away. Anton's reaction is brutal; he orders Geoffrey and Theis to do as they wish with Charlotte. Yet, in a shocking turn, Lizzie, now empowered, poisons the drinks of the men watching Charlotte's performance, leading to their untimely demise.
As the tension culminates, Lizzie and Charlotte confront Anton. With both armed with kitchen knives, they overpower him despite suffering injuries. The film concludes with a haunting yet beautiful cello concert performed by the duo, where Lizzie plays with her left hand and Charlotte with her right, their audience being the disfigured Anton, bound to listen in silence.
