Arnold Schwarzenegger’s new action movie–about Gordy Brewer, a heroic but mild-mannered L.A. fireman whose wife and young son are killed by a bomb blast, inciting him travel to South America to track down the Colombian terrorist responsible for the atrocity–was originally scheduled for release last fall but pushed back after the events of September 11. It was surely the correct decision to hold up “Collateral Damage”–a shot at the end in which the top of a skyscraper explodes in a fireball still stretches the boundaries of good taste after four months–but even had nothing happened in the real world and the picture had shown up on schedule, it would have seemed a terribly dated relic. We’ve seen this sort of thing entirely too often before. To make matters worse, the picture can’t seem to make up its mind about whether it wants to be grimly serious drama (as it is for the initial 90 minutes or so) or comic-book adventure (which is what it becomes in the last half-hour). The result is like several reels from “Proof of Life,” followed by a couple from “Die Hard” to provide a slam-bang finish. The combination just doesn’t jell.

To be sure, on its own by-the-numbers terms, the film is reasonably well made; unhappily, the numbers don’t add up to much. The screenplay by newcomers David and Peter Griffiths does nothing new, but uses the ordinary genre conventions well enough for the first hour and a half (although there’s one coincidence that’s hard to swallow, and the potshots at journalists, politicians and activists are much too cheap). It goes badly off the rails in the final reels, though. The implausibilities pile up wildly (why is a supposedly bomb-filled suitcase empty? how does an explosive-laden truck so easily get into the basement of a high-security building?). A “surprise” twist is terribly strained, and the way in which the hero unravels it is preposterous. And the “Commando”-style action at the close, in which the fireman turns into an invincible champion, goes so over-the-top as to be absurd (as well as, at several moments, unintentionally funny–and, in the unanticipated return of an ostensibly dead character, so ludicrous that it wouldn’t have been out of place in “Friday the 13th” ). Nonetheless, Andrew Davis remains a skilled action director, squeezing as much tension as he can from the mediocre material despite the fact that the special effects in the final reel are pretty poor; it’s not his fault that the result is closer to his “Chain Reaction” than to “Under Siege” or “The Fugitive.”

Schwarzenegger works hard as Brewer, trying in the earlier portions of the flick to project a sense of vulnerability to go along with his righteous anger (there’s also an amusing gag when he “impersonates” a German and is praised for his fluent English). To tell the truth, however, he looks a bit worn for the role (though certainly far fitter than most men his age, and still well-muscled): Gordy is yet another example of the aging Hollywood protagonist who’s happily married to a wife more than twenty years his junior and has a kid of seven instead of thirty. (If things get much worse, the planned “Terminator 3” will have to bear the subtitle: “Even Metal Rusts.”) As good-natured as Brewer’s supposed to be, moreover, he’s still given enough derring-do to satisfy Arnold’s hard-core fans; when it comes down to brass tacks, Gordy might be a kinder, gentler version of the usual Schwarzenegger persona, but he still gets to bite off a foe’s ear, mutter brief threats at the villains (none of which will become a catch phrase), fry an opponent whom he tosses into a computer console, and finish off another with a swift throw of an axe. It’s all part of a firefighter’s usual day, I suppose.

The rest of the cast doesn’t bring much to the party. Cliff Curtis makes a colorless villain, and Francesca Neri isn’t much better as his wife. Elias Koteas smirks and mugs annoyingly as an unprincipled CIA man. And for some reason John Turturro and John Leguizamo show up for what amount to cameos in the Colombia sequences, presumably to lighten the mood; both overplay dreadfully in an apparent effort to conceal the feeble nature of the material they’ve been given. Among the government types loitering in the background, Millie Slavin is noticeable as a particularly enervated Secretary of State–no Margaret Albright, she.

“Collateral Damage” may well be the last in the long line of studio pictures based on the notion that a stalwart American can singlehandedly foil a small army of committed terrorists; after the events of last year, the premise now comes across as hopelessly, even dangerously, naive. One might want to see it, therefore, to experience the dying gasp of a genre that long since ran out of gas. But there’s a distinct downside. Simpleminded and crude in its effects, it’s the sort of picture that’s depressing when one considers all the money and effort that must have gone into it. True, it’s better than the worst of Schwarzenegger’s recent efforts–“End of Days,” for example. But that’s hardly a ringing endorsement.