All the lonely people … where do they all belong? YouTuber Kane Parsons makes his feature directing debut with this icily brilliant and genuinely disturbing conceptual horror film based on his web series, and scripted by Will Soodik. There is something here of J-horror, the V/H/S found footage franchise, Dan Erickson’s Severance and Nathan Fielder’s The Rehearsal. It’s about people walled up in their own memories, imprisoned in endlessly remembered scenes from their past, or miserably perceived versions of their present existences in which they have become caricatures of themselves, gargoyle stars of their paralysed inner world of failure. Or perhaps the action of the film is not metaphorical in this or any other sense, and the “backrooms” of the title simply exist.

Chiwetel Ejiofor and Renate Reinsve give barnstormingly good performances as Clark and Mary; it is the early 90s and Clark is a failed architect, separated from his wife, and an alcoholic who to make ends meet self-hatingly manages a drearily and eerily vast discount furniture store, called Cap’n Clark’s Ottoman Empire. He does dumb TV ads dressed as a pirate while uneasily aware he should be a sultan to make the “Ottoman empire” pun work. He goes to see a therapist, Mary, a sad, gentle person who markets her own self-help audio tapes and is haunted by childhood memories of her abusive mother.

Poor Clark has to actually sleep in his store, in one of the beds in the little “bedroom” tableaux, the strange approximations of people’s actual living spaces. But one day in the huge basement section, he discovers a supernaturally porous section of wall, through which he can walk to discover an infinitely vast secret network of backrooms – strange installation-style areas showing snapshots of what appear to be different versions of reality. And it goes on for ever. Clark finds that getting out of this non-Narnia of non-places isn’t easy, and neither does Mary when she goes in to look for him.

The production design by Danny Vermette is amazing, combining genuine constructions and digital fabrication. With cinematographer Jeremy Cox, they create an ineffably oppressive, crepuscular kind of dead yellowish light, a light that leaks like radon gas from the strip lighting of a million malls, stores and office buildings. Backrooms progressively raises its game towards the big finish with jump scares, squirm scares and tiny shiver scares. There is real fascination in exploring this vast, invisible city state of fear.