A loose adaptation of Carmilla (1872), it revolves around two teenage orphan girls living in a Catholic convent, who unleash a demonic force and become possessed.

The film was released under several alternate titles on home video in English-speaking countries, including Innocents from Hell and Sisters of Satan.

Immediately after the child is born, Lucy begs a hunchbacked gypsy to bring Alucarda to a nearby stone-walled convent inhabited by an order of Catholic nuns, as she fears the devil will claim her daughter.

The hunchback gypsy appears in the room as Alucarda invokes Satan, and the girls, nude, perform a blood ritual.

Meanwhile, while praying, Sister Angélica has a vision of Alucarda and Justine performing a Satanic ritual with the gypsies in the woods and engaging in a mass orgy.

Upon witnessing Angélica's corpse, Alucarda is suddenly overcome with sorrow, and goes into a fit of rage at the base of a burning crucifix in the chapel.

Some scholars, such as Frances Di Lauro, have noted that Alucarda is undergirded by anti-government and anticlerical sentiments that are manifested in the exaggerated idolatry, representations of clerics as tyrants and persecutors, and overt iconoclasm.

The added dimension of visibly transgressive actions taken by two young women against teachings, structure, and moral guiding principles of the church speaks directly to a post-1968 generation that celebrates the body but has lost faith in the society's master narratives.

"[15] Aesthetically, Rodríguez-Hernández and Schaefer note visual references to Francisco Goya's Los caprichos, specifically during the sequence in which Alucarda and Justine engage in the blood ritual with the hunchbacked gypsy.

[18] Green states that, in the film, "the convent becomes a psychiatric domain without psychiatry," a theme also explored significantly in Ken Russell's The Devils (1971).

[18] The tension between science and religious dogma has been noted as another theme, specifically in the final act, during which the logically-minded Dr. Oszek—a man of science—is faced with supernatural occurrences he cannot rationalize.

Many critics have noted its similarities to Joseph Sheridan Le Fanu's early vampire novella Carmilla, especially the romantic relationship between the main female characters.

[24] Though a Mexican production, the film was shot in English, which actors Tina Romero, Claudio Brooke, and Susana Kamini were all fluent in.

[24] Columnist Michael Weldon of the Psychotronic Video Guide wrote that the film was "The strongest, most imaginative, and visual witch movie since Ken Russell's The Devils.

"[9] Mexican filmmaker Guillermo del Toro has also expressed his appreciation for the film and other works from director Juan López Moctezuma.