It’s certainly an ambitious notion to meld “The War of the Worlds” with a message about US immigration policy, especially on a next-to-nothing budget, and you have to admire writer-director Gareth Edwards for attempting it. Ultimately “Monsters” is too obvious by half, but you have to give it credit for trying.

The premise of the movie—which bears considerable similarity to that of “District 9”—is that huge, squid-like and distinctly unfriendly extraterrestrials have taken over a great swath of northern Mexico, which has been quarantined by the military. From their territory the creatures launch assaults on the rest of the country, while US and Mexican forces attack their jungle enclave.

The human protagonists are Andrew (Scoot McNairy), a photojournalist working for a newsmagazine, and Samantha (Whitney Able), the daughter of the magazine’s owner who just happens to be trapped in Mexico during one of the creature attacks. His boss orders Andrew, a sharp wheeler-dealer after a scoop, to abandon his usual rounds and see to it that Samantha makes it across the heavily-guarded border back to the US.

Unfortunately, events conspire to prevent the duo from using the normal escape routes via plane or ship, and they’re forced to hire a group of grubby mercenaries to lead them northward through the forbidden zone to presumed safety behind the gigantic walls that Washington has constructed to seal the border. Danger lurks every step of the way, of course, and it’s inevitable that even though Samantha is engaged, sparks will fly between the pair, “It Happened One Night” style, as they slip past the creepy critters toward their destination.

“Monsters” gets more than a trifle heavy-handed in dispensing its messages. When Andrew and Samantha climb the ruins of an Aztec pyramid as they approach the border and stare at the defensive line, for example, they muse about how different their country looks from the outside (though when they get to the Great Wall, it proves ludicrously easy to cross—for reasons that provide the last-act twist as they get to the other side). And while both McNairy and Able are decent, neither is exceptional enough to raise their characters beyond the one-dimensional, and real chemistry never develops between them.

It has to be said, though, that Edwards uses his locations well to create a gritty, grubby atmosphere, and he gets realistic turns from the supporting cast. Despite the meager budget, moreover, the effects are surprisingly solid, showing that you don’t need two hundred million bucks to pull off such visuals anymore, particularly if you’re willing (and able) to employ shadow and obstructed perspectives to conceal the limitations.

The essential question raised by Edwards’ picture, of course, is who the true monsters are—the outer-space creatures that wreak such havoc, but (as the ending makes clear) have the same needs and lusts that we do; or the humans who, with some exceptions, take advantage of one another whenever the situation allows and draw barriers among themselves. That’s the sort of humanistic issue the best science fiction often addresses. But as “Monsters” shows, merely raising such matters isn’t enough; they have to be treated in subtle and imaginative fashion, and Edward’s film doesn’t meet that standard.

Still, the fact that it has something on its mind makes “Monsters,” for all its flaws, more interesting than a brainless, bombastic big-budget bore like “Cloverfield,” At least it has some smarts, and uses its modest budget to a purpose. And if it doesn’t achieve everything it sets out to do, it earns points for the effort.