Green Room Anton Yelchin, Alia Shawkat, Patrick Stewart. Directed by Jeremy Saulnier. Rated R. Opens Friday in select theaters.

At its core, the plot of Green Room is pretty basic: A group of innocent people witness an act of violence they weren’t supposed to see, and they have to escape from the criminals who don’t want to leave loose ends. What makes Green Room a great movie is the way writer-director Jeremy Saulnier adds specific details to that basic scenario without ever letting up on the narrative momentum. There are no distractions, nothing that doesn’t contribute directly to the near-constant peril the main characters are in, but the movie never feels like a generic cat-and-mouse thriller.

It helps that the antagonistic factions are drawn from subcultures that are so readily identifiable: If Green Room were a direct-to-VOD cheapie, it would probably be called Punks vs. Neo-Nazis, and Saulnier does a great job sketching both ultra-indie punk band The Ain’t Rights (so punk that they don’t even have a social media presence) and the scuzzy rural white power lodge where they find themselves playing a gig after an earlier show has fizzled out. This movie could have just been about the weird combination of culture clash and camaraderie between the left and right wings of underground hard rock, and it would have been fascinating.

But it’s really about what happens after the band’s set, when they come across a dead body backstage and are no longer allowed to leave the venue. Led by the quietly menacing Darcy (Patrick Stewart, using his typically regal manner for evil), the neo-Nazis are determined to snuff out these inconvenient interlopers, but Ain’t Rights bassist Pat (Anton Yelchin) leads his fellow musicians (plus another hapless witness, played by Imogen Poots) in a surprisingly effective counter-siege.

Green Room doesn’t have the emotional resonance of Saulnier’s brilliant 2014 revenge thriller Blue Ruin, but it has the same matter-of-fact brutality, an unflinching look at the devastating consequences of violence applied by people who don’t really know what they’re doing. Once the events are set into motion, no one on either side can really stop them, and every edit, camera movement and line of dialogue in the movie propels it toward its inevitable bloody end.