It's the same story you've seen a thousand times: Boy gets Girl; Boy loses Girl; Demon Monster from Hell shows up; Boy tries to rescue Girl.
I'm sure I am not the first to point out: this movie is a combination of The Blair Witch Project and Godzilla. It follows a bunch of twentysomething Manhattanites around, first at a tedious party. But soon, five of them—oops, sorry, four of them—are on a nightmarish run from a very ill-tempered creature.
It's a kind of love-it-or-hate-it kind of movie, with me kind of loving it, Mrs. Salad kind of hating it. The protagonists are narcissistic and whiny. The movie is imagined to be shot from a single video camera they cart around with them. As the Rifftrax guys pointed out: it looks more authentic because it's poorly shot and the sound is lousy.
But to be fair, the special effects are impressive; given the premises, you have to admit, yeah, the results would probably look just like that.
It's short at 85 minutes, and they could have trimmed the party scenes more than a bit.