Ginny Gordy-Dabrowski and her second husband, Dave Dabrowski, send Ginny's troubled teenage daughter, Katherine 'Kit' Gordy, to the mysterious Blackwood Boarding School. Upon her arrival, she is greeted by the peculiar headmistress, Madame Duret, and the stern housekeeper, Miss Olonsky. Kit quickly notices an ominous door at the end of a dimly lit hallway, which Madame Duret warns is strictly off-limits.
Kit soon bonds with other troubled girls, including Ashley, Izzy, Sierra, and Veronica, who frequently cross paths with her. After confining everyone's cellphones, Madame Duret insists that her strict regulations ensure each girl reaches her full potential. Subsequently, Miss Olonsky begins administering nightly medication to the students.
The girls encounter the school's faculty, including Professor Farley, Dr. Heather Sinclair, and Jules, Madame Duret's attractive son. In addition to her role as headmistress, Madame Duret teaches art, guiding the girls as their unique talents begin to flourish. Ashley quickly rises as a gifted poet, Sierra showcases immense artistic ability, and Izzy excels in mathematics. Meanwhile, Kit uncovers her own extraordinary talent for playing the piano.
During a private piano lesson, Kit confides in Jules about the heart-wrenching trauma of losing her father at a young age. A mysterious incident occurs when Kit blacks out outdoors, only to find herself abruptly reappearing in the music room. Ashley shocks everyone as she shares her eerie vision of a woman named Elizabeth, whom she claims inspires her writing. Kit's suspicion grows when she glimpses a shadowy figure lurking outside Ashley's door, prompting the girls to spend the night together for safety. Kit opens up about her father again as they all wonder why they were selected for Blackwood.
The following day, Kit tells Madame Duret about their sights, but the headmistress dismisses them as mere side effects of the medication, which she intends to stop administering. The girls, except for Veronica, continue to shine in their studies. Madame Duret reprimands her son after he interrupts an intimate moment with Kit.
As Kit takes a closer look at Sierra's artwork, she notices the initials T.C., mirroring those on several paintings in Madame Duret's studio. Ashley asserts that she feels a presence overtaking her during her writing sessions. Through a library investigation, Kit discovers that deceased painter Thomas Cole seemingly possesses Sierra, while poet Elizabeth Webb and composer William Kessler occupy Ashley and Kit, respectively, during their creative pursuits.
Kit also begins experiencing unsettling visions, such as bats crashing through a window and specters chasing her. She and Veronica decide to investigate the foreboding room at the end of the hall, uncovering files that reveal similar cases where girls were driven to madness and suicide under questionable circumstances. However, Kit narrowly escapes after Veronica is captured by Madame Duret and Miss Olonsky.
As Kit is drawn toward a vision of ghosts surrounding William Kessler at a piano, she yields to the urging of Madame Duret and her instructors and plays beautifully. A disfigured spirit demanding to be heard disrupts her reverie. Realizing the gravity of the situation, Kit gathers her fellow students and teachers to the main foyer, where Madame Duret unveils a shocking truth: the girls possess the extraordinary ability to channel geniuses who perished young, offering them a chance to finish their unfinished works. Madame Duret promises Kit a connection to her father if she chooses to follow this path.
Tragedy strikes as Kit discovers Ashley attempting to end her life. Ashley ultimately leaps from her window, crashing through the greenhouse roof. With a heavy heart, Jules assists Kit by providing keys to summon help by unblocking the cabinet with their confiscated cellphones. A chaotic race ensues as Kit finds Sierra's lifeless body and discovers Izzy captivated by a mathematical equation, fighting to bring her back to reality.
In a confrontation with Madame Duret and her staff, Kit reveals her call for help, inciting Madame Duret's wrath for Jules' betrayal. Miss Olonsky intercepts Kit, locking her in a room where she finds Veronica possessed and ensconced in a circle of candles. Despite the gathering ghosts, Kit urges Veronica to resist, leading to her liberation, albeit at the cost of knocking over a candle.
As the building is engulfed in flames, Izzy remains spellbound and unwilling to leave. Kit and Veronica witness Miss Olonsky prepare to kill Jules, but Dr. Sinclair intervenes, only to be overwhelmed by ethereal entities as the girls abandon the chaos. An apparition confronts Madame Duret, who inadvertently stabs Professor Farley amid the haunting.
Recovering from her earlier shock, Miss Olonsky follows the ghost of her deceased son, leading her to the greenhouse where she meets her demise in a sudden explosion. Meanwhile, Kit chases the vision of her father, guiding Veronica to safety outside. However, she and Jules encounter the final moments of Blackwood, where flames swallow Madame Duret.
In the midst of this horror, Jules heroically sacrifices himself to shield Kit from a collapsing beam. As paramedics and authorities arrive, Kit collapses, resulting in a closing journey with her father's ghost that offers her peace before she awakens in an ambulance, ultimately returning to life.
