Before I get too deeply into this, I should probably bring up the fact that, for me, the whole 'Conjuring' saga is a bit of a jumble. I tend to see these movies upon their release, then leave them alone, thus forgetting much of what happened by the time the next one comes along. Hell, I still haven't even seen 'The Nun', and the only one I've actually reviewed yet on this site has been 'Annabelle: Creation'. This is a series that I give enough respect to, however, as they tend to be creatively scary spook house movies, having a sort of psychological intensity to them that I rather enjoy.
While this one is no exception, I have to admit that it's one of the weaker of the series, if only because its ideas and concepts are getting stale. By that I mean the whole demonic possession thing, which I have mentioned time and time again in movies like this. If superheroes have oversaturated the action/sci-fi genre, then demons and hauntings have oversaturated the horror genre. Speaking for myself, there was nothing this movie could have shown me that would have been particularly shocking. It's not without a few moments, but there doesn't seem to be anything left to make me go "woah" the way I did when I first say Regan crab-walk down the stairs in the director's cut of 'The Exorcist'.
While all of the 'Conjuring' movies are "based on true stories" from the past, they still flow in order, and this one takes place in 1981; ten years after the first 'Conjuring'. Once again, we follow professional demonologists, Ed and Lorraine Warren (Patrick Wilson and Vera Farmiga, respectively) as they are documenting the exorcism of David Glatzel (Julian Hilliard); an 8-year-old boy who quite honestly does a fantastic job here. Among the family that are with them are his sister, Debbie (Sarah Catherine Hook) and her boyfriend, Arne Johnson (Ruairi O'Connor). During the nightmarish process (and probable love letter to 'The Exorcist'), Arne tells the demon to spare David and pick him instead.
Thing seem to settle down after that, and the demon seems to just fizzle away. However, Ed witnesses the demon actually do the body-jump before it fades, and he's convinced that somewhere within Arne, the demon still lies. One day, while feeling very ill, Arne commits a murder, and this brings us full circle to the title of the film. The Warrens are supportive of Arne, and want to prove beyond the shadow of a doubt, in court, that Arne was possessed at the time. However, when Arne is able to read from the Bible, that suggests he couldn't be possessed, which leads the Warrens to investigate further, uncovering some harsh realities that probably won't hold up in court - but they still have to try, knowing that Arne wasn't to blame.
It seems that all of the 'Conjuring' films are linked to unique, real-life cases involving the Warrens. This namely includes this title, and 'Conjuring 2', which are based on very specific cases. The first seems to borrow from a few different cases, namely 'Annabelle' and 'Amityville' (if I have that wrong, please feel free to correct me). But upon a bit of research, it looks like they have enough material to make about two or three more movies if they carry on with things - the question is, should they? Again, speaking for myself, demonic possession as a whole is just plain stale now, and there are no surprises anymore. Not to say that things are absolutely predictable now, but you're never taken off your guard anymore. What's more is the fact that over the years, the Warrens legitimacy has come into question in a big way, even potentially putting the legendary 'Amityville' story on the chopping block
It's a good time to remember that when a movie says "based on a true story" that does NOT mean "this 100% happened". In horror, it's a great way to get butts in seats, and none did it much better than 'Texas Chainsaw Massacre', whose "true story" aspect was simply that Leatherface was essentially an Ed Gein type. Everything else about that movie just screams Grindhouse entertainment. I'd suggest that's exactly how to take something like this - it's taking the concept of the Warrens and making a decent horror story out of it. I find it best to just roll with things for entertainment value, and not feel the need to point at the screen and say "that never happened in real life" or "oh please, that's impossible". If one can see this as something that simple, it's perfectly passable... but I'd also probably recommend the first two quite a bit over this.