"We're going to need lots of lovely sharp spears," says the teacher to her little pupils as they prepare to bash the bad guys' brains out. "Fortress," a gripping but unsavory item to be seen on Home Box Office tomorrow night at 8, crossbreeds "Straw Dogs" and "Lord of the Flies," with the result a garish mutation that does, indeed, sustain one's interest.

It sustains one's interest the way falling down the stairs sustains one's interest.

Rachel Ward plays a teacher in a remote Australian one-room schoolhouse. The script lingers nary a moment over introductions so as to get right to the jeopardy, represented by a quartet of vicious kidnapers in Halloween masks. The meanest hides behind the face of Santa Claus, a detail that evokes the scandalous feature "Silent Night, Deadly Night," about a Kris Kringle killer. The little children are rounded up, roughed about, carted out into the wild and sealed with their teacher in a cave, while the kidnapers go off, presumably to elicit multiple ransoms.

The caper and the modus operandi seem improbable even for ruthless criminals, but what writer Everett De Roche and director Arch Nicholson want to do is get the thriller wheels in motion, and they do. Putting children in peril is a cheap trick, but it works. You can't relax while these tykes are in danger, and they're in danger through the whole film (a full 20 minutes is spent in that cave).

Where the filmmakers stumble is in passing off violent exploitation as a statement on what beasts we testy humans are. They're trying to have it both ways and should have settled on just having it one.

Like the film, Ward is all surface. She looks so good that you're distracted from the fact that there may be no performance there. The writer tells us nothing about this teacher's background and circumstances, and Ward doesn't elaborate in anything she does. The students are better. As a young woman who has rather emphatically entered puberty, Rebecca Rigg is quite poignant, given the context, and Sean Garlick brings a furtive kind of brio to the role of the most criminally inventive of the kids.

Revenge is surely the most popular TV movie topic of the year. It's being wreaked right and left. The film glorifies the bloodlust with which the kids fight back, pausing once or twice to sense a certain disquieting savagery, then plunging on.

"Fortress" looks like a real movie -- slickly shot, edited and scored -- one that would probably get a PG-13 rating if TV movies were rated. As it is, HBO will preface it with a warning that it "deals with mature subject matter that may not be suitable for all audiences."

The movie is definitely not the kind of humdrum HBO stuff you can drop after 10 minutes to go a-wandering among the channels. On the other hand, it lingers about as pleasantly in the memory as a rough bout with the flu.