A gigging American punk rock band take a wrong turn into most dangerous territory when they accidentally wind up performing at a seedy bar for jackboot wearing Nazis. Well, it’s more Doc Martens and skinheads actually, but not of the nice This is England (Combo notwithstanding) variety. It’s out of the frying pan and into the fire for the poor kids when they stumble across a murder and find themselves trapped in the bar’s green room, beset by violent skinheads determined to shut them up for good.

Punk rockers versus angry Nazis already sounds like an excellent horror movie combo (no, not the This is England guy again), but Green Room manages to cement its must-see status by casting Sir Patrick Stewart as the bar owner and lead Nazi. Stewart unleashes a chilling villainy we’ve not seen since his turn in Mel Gibson’s Conspiracy, being at once reasonable and utterly terrifying, clearly relishing the role. On the other side of the green room door is another Star Trek universe’s Mister Chekov (the younger one - Anton Yelchin) as nice guy bandmate Pat.

It’s your standard softie to superhero arc for Pat, beaten and battered until he learns to fight back and unleash a little Inglourious Basterd on these Nazi nasties, assisted by his friends (including Arrested Development’s Alia Shawkat) and fellow witness Amber (Imogen Poots). No vampires, but Green Room is like a grungier From Dusk Till Dawn, with relentless action and vicious bouts of shocking violence breaking up the tense barricade sequences. Obviously our aged Patrick Stewart doesn’t get too much of the action, instead sending in wave after wave of burly thugs, shotgun wielding bikers and slathering attack dogs to deal with his unwanted guests.

The characters aren’t exactly fleshed out, but they’re likeable and sympathetic enough that it hurts to see them brutalised and murdered, just as there are genuine fist-pumping moments to be had as they get to fighting back. A pitch black streak of humour stops it from getting too depressing, even as the violence gets increasingly more brutal and upsetting. As in her appearance in The Final Girls, Shawkat’s presence here is like a shortcut to my heartstrings, her baby face (those freckles!) and Arrested Development history making one come over awfully protective of the actress and her character. Similarly, one can’t help but root for the always sympathetic Yelchin and adorable Poots. That likeability extends to its villains too, Stewart’s usual genial demeanour bleeding into his Darcy Banker, making him an oddly charismatic Nazi bastard. Green Room is cannily cast, and this allows the writing and action to cut directly to the chase.

Writer/director Jeremy Saulnier marked himself as one to watch with his Murder Party and Blue Ruin, and Green Room doesn’t let down the streak. It’s more conventional than Murder Party and less artful than Blue Ruin, but it is his crowd-pleaser, almost more action film than horror movie. Fans of fast-paced siege movies like Assault on Precinct 13 and From Dusk Till Dawn should go wild for this one, which deftly sidesteps its undeniable unoriginality and occasional predictability with its style and punk enthusiasm.

Brutal and unsettling yet also triumphant and fun, Green Room is an early contender for one of the year’s best movies, ably bolstered by a killer performance from one of our finest actors (and some very good young ones too). A vicious punk blast, it’s anarchy with a heart.