One of the handful of woman directors at Venice, Ana Lily Amirpour made her mark originally with her much-liked directorial debut A Girl Walks Home Alone at Night and its smart pitch: “The first Iranian vampire western.” Now she has completed her follow-up, and The Bad Batch treads the same genre path – albeit on a more expensive (and expansive) scale, and with a high-calibre cast.

Leading the way is Suki Waterhouse as Arlen, an apparently wholesome individual who is tossed out of the United States into the Mexican desert in the film’s opening scene: she is one of the unwanted “bad batch”, inferior citizenry who are no longer wanted. After briefly wandering the arid flats, she is kidnapped and taken to an encampment called The Bridge, filled with bulked-up steroid abusers; in short order she is chained up and two of her limbs severed for food, like a bunch of other runaways we see sharing the same fate. (The Bridge’s ostentatiously placed American flag is a none-too-subtle indication we are in symbolic territory: the prosperous US is to be represented by narcissistic sadists who consume the helpless.)

It’s a brilliant opening sequence: unwatchably nasty, effortlessly filmed and freighted with ominous meaning. Arlen escapes thanks to a Fisher King-style hobo wandering the wilderness (played by Jim Carrey), and she ends up in a second camp, called Comfort, a place of equally metaphoric implications. Comfort houses the real outcasts – immigrants, the mentally ill, the disabled – but on first inspection at least, appears to have rough charm and some form of rubbing-along livability. (An equivalent, clearly, for your hardscrabble but colourful neighbourhood ghetto.) Comfort, however, is controlled by a goofily sinister cult leader, who exhorts the inhabitants to “follow the dream”; he is played by Keanu Reeves, on excellent deadpan form and looking like the long-lost brother of Ron Burgundy.

Having set up all this portentous, if somewhat cumbersome, social-comment weaponry, Amirpour unfortunately allows it to dissipate somewhat. Instead of the parable about ideological forces that it promises, the film evolves into a slightly sappy hunt for a missing child. Arlen kidnaps a rabbit-loving tot from a Bridge-dweller while she is scavenging in a rubbish tip; beefy Jason Momoa, equipped with a giant meat cleaver and an impenetrable accent, comes looking for the kid. Arlen forgets about making common cause with Comfort and its inhabitants to bond with Momoa and help him track his quarry.

It’s a shame, because The Bad Batch is expertly constructed and looks great: Amirpour has presumably been inspired by Alma Har’el’s documentary Bombay Beach, and has picked up the same atmosphere of bone-dry decay as well as using the actual place as a location. Waterhouse, presumably tricked out with CGI amputations – rather like Marion Cotillard in Rust and Bone – makes a plausible desert warrior, calling to mind Rose McGowan’s Cherry Darling in the Robert Rodriguez half of his and Tarantino’s exploitation homage Grindhouse. But the film doesn’t quite live up to its creepy, savage opening, or carry through its best ideas.