THERE’S power in feeling unsettled and this efficiently made, atmospheric, post-apocalyptic survival horror squeezes every morsel of that feeling out into the open while shrewdly keeping us in the dark.

It’s set in a world that appears to have been ravaged by an unnamed but very deadly virus that very quickly reduces its victims to laboured breathing and boil-laden skin until they succumb to death.

Holed up in a secluded and highly secured house in the middle of the woods, Paul (Joel Edgerton) tries desperately to protect his wife Sarah (Carmen Ejogo) and teenage son Travis (Kelvin Harrison Jr) and retain some semblance of normality in a terrorised world. But his safety and ability to trust is tested when a mysterious stranger (Christopher Abbott) arrives seeking refuge for his own family.

Writer-director Trey Edward Schults plays his cards close to his chest, choosing claustrophobic tension over giving us any definitive answers. The film evokes everything from Jim Mickle’s post-apocalyptic horror Stake Land and underrated dirty bomb thriller Right at Your Door, to name but a couple, but with a keen sense of its own identity.

It eschews cheap jump scares and gimmicks for more old-school horror techniques; slow and steady journeys down dimly lit hallways to dreaded doors that dare not be opened, noises going bump in the night, horrifying dream sequences upon dream sequences and a score by Brian McOmber made of elongated violins and violent piano chords.

There’s a refreshing, classic simplicity about the film that’s quite effective indeed.

Despite the main danger being a virus, there are no walking dead shuffling from between the seemingly endless trees. That threat is used to induce and play on paranoia of both the characters and the audience as we come to care about them, thanks to some effectively understated performances, particularly from Edgerton, left, as a father doing his best to be strong in a world that’s set to reduce everyone to a mess.

It’s a horror film about human behaviour in heightened and up-close-and-personal situations, the ability for one person to trust another in an extraordinarily bleak time when survival and looking after one’s own is of the highest priority. Schults exposes that theme and leaves it pulsing like an infected wound. It will leave you thinking “what would I do in that situation?” as much as it will jangle your nerves.