Most sequels are content to just repeat whatever was good "” and sometimes bad "” about the first film under the assumption that what the audience wants is merely more of the same. When a sequel gets uppity, it brings something new to the party and improves on the original, hence "The Godfather: Part II" and "Bride of Frankenstein."

"Paranormal Activity 2" is an odd little duck, because it improves on what audiences liked so much the first time around, which are the inexplicably loud noises and pop-ups, but it also adds a new element, which is an explanation of what is going on.

The story actually begins as a prequel, and if you haven't seen the first film you may be lost at sea, or at least afloat in the deep end of the pool. Odd things are happening at the upper-middle-class home of Kristi (Sprague Grayden, TV's "24"), Daniel (Brian Boland, "The Unborn") and their teen daughter, Ali (Molly Ephraim, "College Road Trip"). It begins with the standard poltergeist stuff "” inanimate objects that move on their own, strange noises, the family dog barking seemingly at nothing. All this appears to be centered on new baby Hunter.

Kristi's sister and her boyfriend, Katie and Micah (Katie Featherston and Micah Sloat, both from the first movie), pay a visit, and the sibs hint around that this kind of weird shinola happened to them when they were kids. Katie advises Kristi to ignore it so she "won't end up like Mom."

Sinister foreboding.

Kristi and Daniel have security tapes to examine because they had cameras installed to record everything that happens in and around the house after an odd break-in.

Or so they imagined. The inside of the house was trashed, but nothing was stolen. A moment of pre-haunting haunting? As with the first film, we get the story through these tapes and camcorder footage shot by family members.

It's all pretty effective stuff, and director Tod Williams ("The Door in the Floor") delivers at least three seat-dampening jolts that levitate the audience a couple of inches. And these are real scares "” no cats jumping into the frame or friends standing behind the door. As you watch wide-angle shots of normal rooms in a normal home, you may be startled to discover just how many places there are in which nasty surprises could lurk.

One of the best things about the original was the lack of reason for all this spooky stuff to be happening. A big weakness this time around is not just that we are offered an explanation, but that it is a ridiculous one. It exists more to set up a third movie than anything else, but it's groan-inducing.

That said, "Paranormal Activity 2" is easily the most effective creep show of this Halloween season so far. Scary date movies don't come much better. "”Doug Bentin