When it comes to mockumentary type films, there are basically two kinds: good and bad; there's just not a lot of middle-ground with this particular type of filmmaking. Paranormal Activity, which showed at Slamdance, the wild and crazy drunk cousin to the Sundance Film Festival, falls squarely into the "good" camp -- particularly if your definition of "good" includes "will scare the pants off you" and "I had to sleep with the lights on after watching it."
The central idea of the film is that it purports to show actual footage of, well, paranormal activity, in the home of the two protagonists, Katie and Micah, who are living their normal lives until weird things begin happening in their home. Katie, who believes she's been haunted by an invisible, malevolent being since childhood, fears it's followed her to her new home. Micah isn't quite convinced there's anything unexplainable going on, but he purchases a video camera to record their room at night, in an attempt to capture on film any paranormal activity and try to make sense of it. When the camera actually does capture some weird happenings, Micah is at first rather excited by what they have on film; as things escalate, through, both Katie and Micah fear that the entity haunting Katie could turn violent -- or even deadly.