Lucy Klavel, Chloé Coulloud, is a young woman beginning her practical training as an at-home nurse under the careful guidance of the stern, older caregiver, Mrs. Catherine Wilson Catherine Jacob. The last patient on their rounds lives in an old, remote mansion, a place thick with shadows and stuffed animals. When Lucy enters, she finds Mrs. Wilson tending to the bedridden Deborah Jessel [Marie-Claude Pietragalla], a once-renowned ballet teacher who has become a captive to the mansion’s mysteries. A rumor swirls through the halls about a hidden treasure of gold and jewels somewhere on the property. After her first day, Lucy confides the tale to her boyfriend, William [Félix Moati], and together with Will’s brother Ben [Jérémy Kapone], they decide to search for the rumored treasure on Halloween night.
They gain entry through a basement window and step into a room crowded with stuffed animals gathered around a tea table, where a single locked door awaits. Lucy correctly suspects that a key worn by Deborah Jessel is the key to this door. Behind the door, a white sheet covers what looks like a stuffed mannequin of a young girl. Lucy believes this is the corpse of Jessel’s mute daughter Anna, still dressed in a ballerina outfit. When she twists the key on the pedestal the corpse stands on, it starts to spin slowly as if a music box were playing a haunting ballet.
noises from above grow louder as they scramble to escape, but the window Lucy entered through is now secured with iron bars. In their attempt to flee, the trio becomes separated. Ben finds himself in an ominous operating room with no visible door or explanation and is killed by veiled ballerinas who appear from nowhere.
Lucy, meanwhile, encounters Mrs. Jessel seated at Anna’s old tea party table. A flashback reveals Jessel’s brutal past as a ballet teacher: a girl leaves a class, and later Jessel discovers another girl dead in Anna’s room; Anna is shown hovering over the dead girl, drinking blood. It becomes clear that the Jessels are vampires.
William is attacked when Ben, now a vampire, closes in on him. Will stabs Ben with a pair of scissors, but Mrs. Jessel attacks and kills him. Lucy learns that Anna is not actually dead. Anna attempts to drink Lucy’s blood, but Lucy pushes her away. Mrs. Wilson enters, attacks, and sedates Lucy.
Under the watchful eye of Wilson, Mrs. Jessel implants moth pupae into the throats of Lucy and Anna—a ritual meant to exchange their souls. Jessel believes she has succeeded when Anna awakens with a changed eye color, now inhabiting a new body. Jessel orders Anna to dance, but the girl resists. Wilson tries to punish her, yet Anna stops her by driving a pair of scissors into Wilson. Anna then turns on Jessel. The two women fight, with Jessel bleeding and still trying to bite Anna, until Lucy lashes out to force Wilson to release Anna, and the two girls push Jessel off a third-floor balcony.
Lucy and Anna escape the mansion together, walking along a seaside cliff hand in hand. Lucy, now in Anna’s body, leans over the edge and seemingly flies into the sky, while Anna, inhabiting Lucy’s body, watches as Lucy’s scars fade and vanish.
