SOMETHING WIKUS THIS WAY COMES: Sharlto Copley has a bad day.

They didn’t come to make war, but they didn’t necessarily come in peace, either. These beings from beyond didn’t speed through space-time to probe us, or to Hoover our brains for food, or to establish an interstellar federation. The aliens in District 9 simply ran out of gas on the wrong side of the universe, and they only want to go back home, wherever or whenever that is. Marooned in Johannesburg, South Africa, on cruel and xenophobic planet Earth, the so-called “prawns”—a human slur that happens to be pretty accurate—actually have quite a bit in common with the earthlings who’ve shunted them into the filthy slum-city that gives the film its name: technologically advanced enough to skip through the cosmos, but sadly hapless and discombobulated once they lose the map. It is an unfortunate frailty that District 9 itself shares, and that first-time director Neill Blomkamp can’t quite overcome.

In an interview with slashfilm.com, Blomkamp said he wanted to make a film that “didn’t depress the audience and kind of ram a whole lot of ideas down their throat that maybe they didn’t feel like hearing.” Could there be a more disheartening statement of purpose by a young artist, or a more cynical underestimation of an audience’s intelligence? Blomkamp’s admission of needless compromise is especially baffling in light of District 9’s first act, 20 brilliant minutes of faux-documentary dread. It’s not subtle, nor is it half as politically astute as Blomkamp seems to believe, but it is a mini-masterpiece of harrowing and darkly funny filmmaking.

A frenetic collage of talking-head interviews, shaky man-on-the-ground footage and fuzzy news reports establishes the alien problem—they’re here, they’re weird, we can’t get used to it—and the South African solution: relocation of the prawns from District 9 to an internment camp away from humans. Spearheaded by Multi-National United, a nefarious government-sponsored operation with an interest in alien weaponry, the forced migration gets whipped into overzealous shape by earnest bureaucrat Wikus Van De Merwe (Sharlto Copley), who is equal parts buttoned-down doofus and casually sadistic alien-hater.

Blomkamp’s deft meshing of cinéma vérité and seemless CGI attains an unsettling apotheosis in District 9’s early scenes. The simulation of documentary style, familiar and comforting to our news-soaked eyes, becomes a delivery system for outlandish visions and uncanny panic. It’s like watching an old home movie only to find your beloved grandmother has been replaced on tape by a giant tarantula. Sickly prawns and gruff soldiers scuffle in the dusty streets, Wikus gleefully torches an alien hatchery, and Earth’s accidental visitors, wrapped in ill-fitting human clothes, scavenge through heaps of garbage. It is nightmarish stuff—Cloverfield minus the dopey dialogue—and Blomkamp is a skilled conjurer.

But when Wikus goofs mid-mission and squirts himself in the face with a mysterious alien ooze, everything falls apart. Not only does Wikus’ body begin a Fly-like transformation, complete with Cronenbergian ejections and suppurations, but the film itself begins to deteriorate. What began as a sharp descent into hell becomes a timid tour of action-film clichés. This is where Blomkamp begins withholding the “whole lot of ideas” we’re apparently not ready for. Instead, as Wikus flees Multi-National United, thereby frustrating its sinister plans for his rapidly hybridizing body, Blomkamp marshals a sheepish parade of barking villains, gray-matter splatter and soft-headed sentimentality. Yes, aliens cry. And yes, those lachrymose bug eyes are somewhat affecting. But no, you didn’t pay for a Spielberg film.

District 9’s skid into rote convention is matched by a retreat from the documentary conceit that so expertly delivered us into this strange world in the first place. Gone are the dread-spiked familiarity, the hijacked normalcy. In their place is a visual spectacle of hectic ambivalence not unlike every other gutless extravaganza you will see this summer. Oh, there are guts, I guess, but they are flung at the screen to distract you from Blomkamp’s loss of nerve. I have faith that Neill Blomkamp will one day make a film that’s a marvel from beginning to end. The question is whether he can muster the necessary faith in us. We’re smart enough for you, Blomkamp. Take your best shot.