The character of the demonic nun appeared in The Conjuring 2 and this is the spin-off prequel. It's 1952 and the Vatican sends a troubled priest, Father Burke (Demián Bichir), and a nun-in-training, Sister Irene (Taissa Farmiga), to Romania to investigate the apparent suicide of a young nun. Once they reach the abbey it becomes clear that an evil presence consumes the place, and this evil takes the form of the demonic nun.
So, a malevolent force is somehow unleashed and then must be contained. That is the simple premise of The Nun, but the movie is generally slow moving and fairly dry in its characters and situations. Burke and Irene are thinly drawn and extremely dull, and it seems that the actors know it, as they appear very bored on screen.
This movie sorely needs an adrenaline rush like the craziness of, say, The Evil Dead (1981). However, there is an early set piece in a cemetery which is wicked fun, even throwing in a nod to the coffin piercing scene from Lucio Fulci's otherwise rotten City of the Living Dead (1980).
After that, though, there are just endless scenes of characters being thrown against walls by an unseen force. There are so many “scares” one after another that most of them are rendered ineffective. Besides, when I see a floating nun, I instantly think of an early scene in The Blues Brothers (1980) where Jake and Elwood visit Sister Mary Stigmata.