The film is loosely based on Sheridan Le Fanu's novel Camilla, with subtle references to Marquis de Sade's Justine.
In the 16th century, Alucarda lived in a former monastery in Mexico that doubled as a hospice.
Orphaned at birth, this mysterious young woman lived a normal life until her teenage years, when Justine, whose parents had just died, arrived at the convent.
They immediately connected and became friends. Their innocent relationship turns to lust and ends when Alucarda unknowingly visits the horrible place of her birth;
Later, on the advice of a gypsy, she summons Satan and makes a blood pact with Justin in a pagan ritual that unites them until death.
From that moment on, Arlucarda and Justine will completely change their behaviour; with blasphemies constantly coming out of their mouths, the nuns and priests plan to perform a forced exorcism.
However, the attempt to save their souls will only make the situation worse.