I guess this is sort of a spoiler post. Though the movie doesn't really hinge on too many plot points, I'd hate to ruin someone's evening by having them hear about it here first. The film's premise is that Micah and Katie have been experiencing strange phenomena, now they're going to videotape it in order to get to the bottom of it. What you're watching is the footage, found and edited by a third party, of the goings on in their house. The psuedo-spoiler is coming up, so you many want to stop reading... now:
It's a demon.
It's announced within the first twenty minutes of the movie, so it may not be a huge spoiler. It appears that Katie has been followed by a malevolent entity since she was eight years old. As things get worse, the question hanging in the air (for the audience and for the players) is, "How long do you stay in a relationship with supernatural baggage?"
I'm going to go ahead and toss another spoiler out there, cause if you've read this far you clearly don't care. Things don't end well for the happy couple. Things really do not end well. But they could have.
I can't find my notes from class to back this up, but I remember my studies in anthropology, mythology, and folklore concerning demon possession. One is at risk for demonic possession when existing in a liminal state. Liminality is the state of being in-between, not having a set identity or social status.
The New Testament holds the story of Legion, one of the more well known stories of demon possession. The setting for the encounter between Christ and the possessed man is a graveyard. A graveyard is a liminal place, a crossing between the land of the living and the land of the dead.
In The Amityville Horror, George Lutz moves his new bride and her children to a spacious house with a dark history. George is the new addition to the family unit, he doesn't yet have a place, and is targeted for possession by the residence's resident wraith. His lacking a definite, fixed position in the group makes him an easy mark.
Now, Paranormal Activity's two primary characters are Katie and Micah, a young, unmarried, though committed couple. When asked about their relationship, Micah says that they're "engaged to be engaged." Thus assigning them to the grey area between boyfriend & girlfriend and fiancees. This puts them in a liminal state.
I believe that had these two kids taken time to figure out their relationship, everyone would've walked away fine. Now, it may seem that applying what I learned in Humanities courses at a State University to a motion picture scenario doesn't guarantee success any more so than the next guy's suggestion. But I can totally prove I'm right on this one.
Even if my liminality argument falls through, taking their relationship to the next level or calling it quits would have thwarted the demon's attempts. Let's say Micah proposes and Katie says, "yes." The Psychic tells them that beings such as ghosts and demons feed off of negative energy. The positive energy introduced by the engagement would give the demon less juice with which to work. On the other hand, after a long and serious talk, they might have decided to just be friends. Yes, this would produce negative energy; however, it would remove another source of the demon's power. The Psychic warns against trying to communicate with the demon, for it may view this as an invitation for further interaction. Micah does not heed this warning. If they split up, Katie wouldn't have to deal with Micah taunting, teasing, and actively seeking out the wickedness surrounding them. The demon would not have a door to step through without Micah's shenanigans.
So, what can we take from this movie? Yes, having a loosely defined partnership not weighed down by societal labels is fun, but it will get you killed.