After the appalling mess of 2004’s “Alien Vs. Predator,” one could only look forward to this follow-up with trepidation. And the fact that it went unscreened for critics didn’t bode well, either. In the event, though, “Aliens Vs. Predator: Requiem” turns out to be far better than its predecessor—not good, mind you, but probably about as efficient an example of an unnecessary sequel as was possible under the circumstances.

Shane Salerno’s script jettisons all the Antarctic, ”Chariots of the Gods” nonsense of its predecessor in favor of a straightforward alien-invasion-with-chase scenario (much as Cameron’s “Aliens” turned Ridley’s gloomier “Alien” into a gung-ho battle flick). A Predator spaceship carrying some Alien embryos (as well as an infected Predator, it seems) crashes near the picturesque town of Gunnison, Colorado and unleashes the parasites upon the local population. After a hunter and his little son are dispatched through the usual chest-bursting method and a deputy skinned alive by a Predator who arrives separately to track down the ravenous beasties, all hell breaks loose as the Aliens multiply and claim more and more victims.

The remainder of the picture falls into two parallel sections. One involves the battles between the Predator and his myriad prey. The other has to do with a cast of human characters who are gradually whittled away through either Alien absorption or the collateral damage from inter-species mayhem. Among the most notable are ex-con Dallas (Steven Pasquale) and his troubled younger brother Ricky (Johnny Lewis); Sheriff Eddie Morales (John Ortiz), an old chum of Dallas; Jesse (Kristen Hager), the blonde coed Ricky lusts after from afar and her bully of a boyfriend, Tim (Sam Trammell); and just-discharged Iraq vet Kelly (Reiko Aylesworth) and her little daughter Molly (Ariel Gade). Some of them survive to the end, but some do not—and the latter have a lot of company among the supporting cast.

“Requiem” has some virtues. One is that co-directors Colin and Greg Strause (billed as “The Brothers Strause”) actually show considerable facility in generating tension, choreographing action scenes and using sudden shock effects. Another is that by and large the actors are less amateurish than one usually finds in such flicks, though none of them could be described as Oscar-worthy. (It’s also a nice nod to give the hero played by Pasquale the same name that Tom Skerritt, the captain in “Alien,” had.) Some of the CGI work is pretty good, and the scenery is nice (and generally well caught by cinematographer Daniel C. Pearl).

Most importantly, though, the script takes some unexpected turns, especially in terms of the selection of victims. People you might not expect to get terminated do—certainly it’s unusual to find a child one of the first people killed, a good sign that a horror movie wants to be taken seriously (the same thing happened in Guillermo del Toro’s “Mimic”); even infants in a hospital birth ward prove fair game. And generally the picture avoids the abundance of gore so many such flicks wallow in nowadays—the deaths here are surprisingly chaste, and the bloody moments more subdued than the norm. It also eschews a tongue-in-cheek tone, preferring instead to keep things serious. (The only obvious gag, sure to get a cynical laugh from many viewers, come when a motley bunch of survivors are told by army authorities to head for the center of town to await evacuation, and one of them objects that the plan seems designed to insure their doom rather than save them. When one of the group replies, “The government wouldn’t lie to us,” it’s clearly intended as a joke.)

On the other hand, much of the monster stuff is depressingly video-gamish, and the final battle between Predator (Ian White) and Alien (Tom Woodruff, Jr.) looks rather like a fight one might encounter between two rubber-suited guys playing Godzilla and Mothra in an old Japanese movie. There are entirely too many chest-bursting sequences, and too many repetitions of that iconic moment when a human turns around to find an Alien slobbering in his face before ripping it off. When these devices are used so repeatedly, they quickly lose their impact. And as so often happens, the picture isn’t very good at clarifying the topography of the battlefield, or giving some indication of how many Aliens there are for the Predator to exterminate. (They also seem to be much less indestructible than they once were—even a pistol is capable of downing them.)

Still, though it won’t ever earn the classic status of either “Alien” or “Aliens,” and doesn’t even match the much less interesting original “Predator,” this surprisingly old-fashioned town-under-extraterrestrial-siege tale is pulled off more effectively than one might have anticipated; even the downbeat “sacrificial lambs” denouement, though predictable, is pulled off briskly enough to avoid getting too heavy. A pity it’s followed by a coda that suggests if there’s another installment, it will go off in a rather different, much less promising direction.