Mexico, 1767. A priest is commissioned to investigate a series of murders that have taken place inside a convent. The sole witness, a nun apprehended by the Inquisition, declares she has hellish visions and contact with the devil. After several interviews, where step by step the massacre is pieced back together, the clergyman is able to glimpse the truth of the facts, which plunges him into a dangerous spiritual dilemma.

