I hadn't intended to watch this movie until a friend told me what a incisive metaphor it was for modern America – the same chatter I'd heard when it came out but had dismissed as the work of a distributor hoping to drum up interest. And to be honest, it's only the premise that has any metaphorical weight. The story is just a mildly effective survivalist thriller.

It's the near future and any and all crime is allowed for a single night of the year. The reason is to let the nation blow off some collective steam and get back in touch with its primal instincts in a politically genteel world where Americans have taken security and civility to extreme and ridiculous degrees.

The subtext enters it because the rich, privileged white suburbanites like the picture-perfect Sandin family the story centres on can afford the shuttered doors and iron bars to keep out the gangs who roam all night looking for violence to do, while those who can't have to try to just stay alive.

One of them is a man credited only as Bloody Stranger (Edwin Hodge), who the Sandin boy Charlie (Max Burkholder) sees outside after the family goes through their annual lockdown procedure. Pleading for help and already hurt, the man touches a nerve in Charlie, who opens the shutter and lets him in. Charlie's parents are horrified, especially his company man Dad and security system salesman James (Ethan Hawke). His mother Mary (Lena Headey) is less enthusiastic about The Purge, and his teenage sister Zoey (Adelaide Kane) is horrified as such a barbaric practice.

But the hurt man Charlie harbours in his bedroom is their least of their worries when a group of masked youths descend on the house. Their leader calmly rings the doorbell and explains that the man the Sandins have taken is was their quarry, and if they send him back outside to face certain death they'll leave peacefully. If not they'll force their way into the house and kill them all.

As the Sandins set about fortifying their abode, it becomes a funhouse of horrors – all dark rooms, nobody knowing where anybody else is and other horror movie iconography that stretches credibility in a suburban house, to say the least.

It (probably) says something about harbouring immigrants or refugees right along with white privilege, class, the media-fuelled thirst for violence, gun control and even the false niceties we elect to show our neighbours when we really want to butcher them. It feels like those and many more were the political issues writer/director James DeMonaco was thinking of, but none of the story, scares or thrills going on are strong enough to effectively deliver them. But the strongest clue about the subtext might actually be in the title – a purge is something we do to flush away unwanted elements of a system. In the politics of this movie's world, that might refer simply to those who can't afford the security measures to protect themselves from violence (minoroties and the poor).