Father Ambrosio (Cassel) is an austere monk of such unquestionable rectitude and principal that he makes Mother Teresa look and behave like Lady Gaga.
Instructed as a novice by Capuchin monks in 17th century Madrid after being discovered as an abandoned baby, his iron faith is indisputable and his fame as an inspirational preacher attracts rapturous congregations.
However, such is his unflinching piety that he unwittingly condemns a young nun to death when he draws out her confession of sins of the flesh and the abbess casts her into a cell to starve.
At the same time he begins to have vivid dreams about a woman in red and crippling headaches which can only be relieved by the healing touch a young apprentice monk whose face - scarred in fire - is hidden behind a wax mask.
Based on the 18th century Gothic novel by Matthew Lewis, this charts the spiritual and moral descent of Ambrosia, drawn down by the devil in human form inside the walls of the monastery itself.
Cassel is well cast as the virtuous priest and director Dominik Moll resists the temptation to ramp up the action, preferring to keep the devil's work insidious and quietly all-consuming.