The film's opening hour trickles down a revenge narrative: Lucie kills the Belfond family. However, the twist reveals that the Belfonds were not a random innocent family, but active members of a wealthy secret cult. The 'monster' slashing Lucie is not a supernatural demon, but her own psychological manifestation of survivor's guilt over abandoning another victim. When Lucie slits her own throat, Anna is left alone.
Anna discovers the underground complex hidden beneath the Belfond house and finds another emaciated woman with a metal plate bolted into her skull. Before Anna can escape, the cult arrives, led by Mademoiselle.
The second act turns into a grueling psychological experiment: Anna is chained in a cell, systematically beaten, fed slop, and stripped of all identity. Mademoiselle explains that ordinary victims break into madness, but 'martyrs' transcend their pain. Eventually, Anna is flayed alive—her skin completely removed. In this catatonic, skinless state, her eyes fixate upward in ecstasy, indicating she has transcended corporeal suffering. Mademoiselle leans in, and Anna whispers what she sees into her ear. Mademoiselle cleans herself up, meets the expectant cult members gathered at the mansion, pulls out a handgun, tells her assistant to 'keep doubting,' and shoots herself through the head.