Here there be minor spoilers. Ye be warned.
A pastor and his wife have two adorable demon children, and they've got the home videos to prove it.
The dad is the definition of the word goober. The mom is a child psychologist who tries to be, like, 1980s sitcom mom. Jack and Emily almost never speak, choosing instead to express themselves by throwing silverware to the floor, biting kids at school and crucifying the cat.
As with every other "found footage" type movie I've seen, there's one character who just can't stop filming, in this case Goober Dad. The mom uses the camera a little bit, mostly when she's being Child Psychologist Lady and speaking very, very softly to prove that she's professional. Apparently you're not supposed to be able to hear what professionals are saying.
The problem with this movie and, indeed, with all slow burn movies, is that it's a slow burn. Sure, maybe it's more effective than if the movie started with the kids murdering folks (or whatever) but it's so damn predictable:
1) everything seems normal other than the fact that one thing is off (in this case, the kids are abnormally quiet)
2) the one thing that was a little off becomes far more noticeable (the kids throw rocks and kill pets)
3) something crazy happens (the kids have bite marks all over themselves)
4) things come to a head (Mom medicates the kids, Dad exorcises them)
5) the calm before the actual storm (meds and de-demonizing works; kids befriend school chum who they attacked earlier)
6) the storm (the kids weren't "fixed" after all)
Sure, the storm is awesome once you get there, and it would probably be a stupid storm if you didn't have to sit through all that other crap.
The problem is, I've seen all the other crap and I know what's going to happen and it's a trial to sit through all of it when I can already tell you the sequence of events.
Why has nobody tried to reinvent the slow burn horror movie yet?
End of line.