It’s fair to say that this movie isn't exactly what one would call a 'serious' horror film. The story is pretty wild and somewhat silly, but that doesn't mean it can't be enjoyable. The basic idea of this little horror movie isn't particularly realistic, and setting aside the silliness of the plot, it’s definitely worth seeing for fans of slasher movies. It seems, though, that the film didn’t receive widespread praise, and I suspect the disappointment comes from those who were hoping for a classic nunsploitation flick but got a Hollywood-style Spanish slasher instead. Another possibility is that some reviewers thought this movie would be similar to "Darkness," which was written and directed by Jaume Balagueró, who also handled the story here. Considering how much more respected “Darkness” was for its atmospheric and artistic qualities, I wouldn’t be surprised if some people expected something along those lines or at least a few connections with this movie. However, "La Monja" wasn't directed by Mr. Balagueró, so I don't really see why there should be any connection. My advice to anyone expecting anything remotely like “Darkness” is to skip this slasher; it will likely be a letdown. In "La Monja," six young women at a Catholic boarding school are tormented by a cruel nun named Úrsula, who also serves as the headmistress. During one sadistic purification ritual, Sister Úrsula goes too far with her old-fashioned methods and tries to drown Mary, one of the students, in an attempt to cleanse her soul. Her harsh ritual is interrupted when the other girls rebel against Sister Úrsula and kill her. Afterward, they make a pact of silence and throw her body into a pool of holy water. Eighteen years later, Eve, Mary's daughter, begins experiencing strange visions of a vicious nun who appears seemingly out of nowhere. Several people start dying around her, including her mother and some classmates. Following her mother’s mysterious death, Eve travels to Spain with her friends, only to discover that she is destined to confront a terrible fate and uncover an unsettling truth about herself. The plot clearly promises a fun slasher... and that's exactly what we get! With a good selection of well-executed murders and a measured use of CGI, this bizarre horror film manages to entertain while also poking fun at slasher films for their similarities and lack of originality. "La Monja" is a solid example of what contemporary slashers are all about, and even though it’s a Spanish production, it does a good job of imitating some of the best American films out there. The supernatural villain reinforces the idea that the director wasn't aiming for a dark or profound film; instead, he simply wanted to create an entertaining and straightforward horror movie perfect for enjoying with popcorn and a soda. If you’re looking for simple fun, give this evil nun a chance, and she won’t disappoint you with her harsh punishments for those who defy her.

Source on TMDB