This movie works because the writing is real (credit that to writer/director Oren Peli), the actors are believable, and the horrifying aspects are part of our own imagination. "Paranormal Activity" doesn't hit us over the head with gruesome monsters or terrifying demons. What it does do is build slowly to a stunning climax, starting with small pieces of evidence that the house in which Katie and Micah (Katie Featherston and Micah Sloat) live together is actually haunted by spirits aimed at driving Katie insane --- or worse.

Micah and Katie are engaged to be engaged, but their decision to live together is severely hampered by what seems to be the presence of a supernatural being that has followed Katie since childhood. Micah is skeptical, but offers to set up a video camera in their bedroom to capture any paranormal activity that might occur while they sleep. He also uses the camera extensively on a hand-held basis as the two go about their daily routines inside the house.

Virtually the entire film takes place there. At times, the couple leaves their bedroom in the middle of the night, but the still camera remains in place, focused on their bed. Thus we hear what is happening downstairs or off camera, but we don't see it because the camera is sitting on its tripod. Some of the more hair-raising episodes occur when we only hear what is going on. If you think "hair-raising" is an exaggeration, see the movie.

A friend's teenage daughter thought the film was boring until the final few scenes. This only proves that younger moviegoers don't appreciate how a well-made movie is constructed, in this case inexorably building to a conclusion we're not entirely sure we're ready to experience. Every time the movie fades to black and re-appears with Katie and Micah asleep, with the time of day superimposed on the screen, there is a real sense of dread. But then again, the skeptics in us go to these things to see if we can really be scared --- and yikes, does "Paranormal Activity" deliver!