Given the right treatment, things that go bump in the night can be scarier than all the fake gore Hollywood special effects shops can deliver.

Such is the case with "Paranormal Activity," a nifty, thrifty little horror movie that comes just in time for Halloween.

Shot for an estimated $15,000, it has become a marketing sensation taking on the monstrous dimensions of "The Blair Witch Project."

Paramount opened it as a midnight show in a handful of college towns, and Internet buzz has made it a hit that's already raked in millions. It may deal with supernatural phenomena, but the movie has become a phenomenon on its own.

The premise is simple. Katie Featherstone and Micah Sloat (using their own names in the movie) play a San Diego yuppie couple living in a house besieged by strange occurrences.

Doors open and close on their own, strange noises interrupt their sleep, and things turn up where they shouldn't.

Micah, a gadget geek, appoints himself the household ghostbuster and sets up a video camera in their bedroom. He also activates microphones and computers and follows Katie around with another camera.

He thinks "cool stuff is going on" while the distressed Katie just wants the weirdness to just go away. When a psychic is called in, Katie confesses that she's been stalked by an invisible presence since she was 8.

The psychic informs the couple that there aren't any ghosts in the neighborhood. What they're dealing with is a demon on Katie's trail. Katie can run but she can't hide.

And above all, he says, don't mess with a Ouija board: It might invite the demon to step up its activities.

Every good horror movie needs at least one idiot, and Micah fills the bill. He's a control freak who thinks he's in charge, and he runs right out to get a Ouija board.

Small cast, a budget that wouldn't cover catering for one day on a regular set and a week's shooting schedule. But then add a savvy writer-director and a couple of good actors.

Filmmaker and video game designer Oren Peli takes a page from Robert Wise's 1963 classic, "The Haunting." He uses sound and the unseen to create suspense. Then he adds some subtle but creepy touches captured by the nocturnal video camera. The rest of the movie's technical realism is captured by the jerky rhythms of a home movie that Peli expertly duplicates.

Meanwhile, Sloat and Featherstone capture the realistic rhythms of a caring couple going through a period of strife. They banter, they cajole, they comfort, they argue.

As the relationship begins to break down, the malevolent presence increases its sadistic bag of tricks, and Peli revs up the sense of impending doom. Judging by the audience reaction at Friday's first show, Peli's own bag of tricks works.