Two years after making a crimson splash in Cannes with his catgut-taut suspenser “Blue Ruin,” U.S. writer-director Jeremy Saulnier continues his grisly journey across the rainbow with the ultraviolent backwoods horror pic “Green Room.” Following a young group of punk rockers as they scrape, shoot and slash their way out of an Oregon neo-Nazi group’s clutches, this wilfully unpleasant midnight special further demonstrates its helmer’s machete-sharp sense of craft, and puts an interestingly matched ensemble — including an outstanding Imogen Poots — gleefully through the wringer. Characterization and emotional investment, however, are in disappointingly short supply, while crucial tension is permitted to dissipate in an anti-climactic final third. Nevertheless, given the right marketing, this dark, dank “Room” could make plenty of green for a genre-savvy distributor.

The Directors’ Fortnight at Cannes is becoming a keenly watched launchpad for commercially viable independent voices in American genre cinema, and the roaring crowd response to Saulnier’s third feature augurs well for a swift, competitive acquisition. Certainly, “Green Room’s” theatrical returns will dwarf the modest $258,000 reaped domestically by “Ruin,” though it’s not the most conceptually ambitious leap forward the director might have taken after that film’s adoring critical reception. Rather, he’s taken this opportunity to demonstrate his ample formal chops and budgetary discipline on crowdpleasing exploitation fare. As a calling card for bigger studio assignments, “Green Room” could prove ruthlessly effective.

As with much contemporary horror by a certain cresting generation of directors, there’s a back-to-basics ’80s sensibility to “Green Room” that goes harder on practical gore and slow-building scares than on severe digital spectacle. Saulnier, however, skews away from the cool John Carpenter pastiche of Adam Wingard and Ti West, taking inspiration instead from a messier strain of what the British term “video nasties.” No icy synthesizers are to be found on a soundtrack cluttered instead with head-drilling punk and metal from the likes of Hochstedder and Syphilitic Lust.

At least some of the music, meanwhile, emanates from the film’s own fictitious band the Ain’t Rights, a struggling punk collective — including singer Tiger (Callum Turner), bassist Pat (Anton Yelchin), guitarist Sam (Alia Shawkat) and drummer Reece (Joe Cole) — introduced in a grievously hung-over state, their van stranded and immobile in a cornfield. Desperate for any gig, they have agreed to perform at a remote skinhead roadhouse in rural Oregon, run and largely attended by white supremacist thugs. Overseeing the proceedings is Darcy (Patrick Stewart, plainly relishing the chance to play unmitigated malice), a merciless gang leader who shares little with his Jane Austen namesake but a propensity for tight legwear.

At the club, the Ain’t Rights — already ill suited to the venue — antagonize the heavily neo-Nazi crowd with a rendition of the Dead Kennedys’ bluntly titled 1981 single “Nazi Punks F–k Off.” Their intention is to beat a hasty retreat, but the night takes a calamitous turn when they happen upon the murder of a young female club patron in the maelstrom. Together with the girl’s oddball best friend, Amber (Poots), they’re targeted by Darcy’s henchmen, who are clinically determined to eliminate all witnesses. In irrational panic, the band and Amber barricade themselves in the backstage green room while they attempt to devise an escape plan. The various, escalatingly bloody permutations, foilings and recalibrations of that plan form the rest of the film, as the skinheads come at their captives with a wince-worthy selection of weaponry and attack dogs. As the body count rises, the survivors are forced to get more resourceful, while the diminutive, drawlingly cynical Amber emerges as the group’s unexpected powerhouse.

This punched-up cat-and-mouse game is luridly entertaining for a stretch, though the fun begins to pall at the 60-minute mark, as it becomes clear that no new principals or obstacles are arriving to shift the stakes of what, beneath the extremist trappings, more or less amounts to a simple house-of-horrors narrative. While “Blue Ruin” uncovered seethingly complex human impulse and psychological subtext under its elemental revenge tale, “Green Room” leaves no such subliminal resonance when the ride is over. While played with sparky gumption by the actors, the characters here are largely cyphers — as, indeed, is Stewart’s chief villain, who’s given markedly short shrift in the film’s latter half. At least Poots, with her frenzied physicality and pointed line readings, suggests an inner life for her character beyond the claustrophobic confines of this story, though she’s hardly an open book.

Expert below-the-line contributions help to maintain a grimy, dry-mouthed atmosphere of unease even when Saulnier’s script and Julia Bloch’s otherwise snappish editing loosen the tension in the home stretch. No longer working as his own d.p., the director has found a sharply attuned ally in Sean Porter, who articulates the cramped spaces and bleary economy of light in the club (given a near-perceptible stink by production designer Ryan Warren Smith) with aplomb. Jessica Needham’s slaughter-happy makeup team, meanwhile, can take credit for a range of nightmare-ready images left in “Green Room’s” red-footed wake.