In Backrooms, we meet Clark (Chiwetel Ejiofor), a functioning alcoholic who, after losing his home, now sleeps in the furniture store he owns, Captain Clark's Ottoman Empire. He visits a therapist, Dr. Mary Kline (Renate Reinsve), but their sessions do little to stop Clark from blaming his ex-wife for his many troubles.
After discovering a door in his basement that leads to a seemingly endless labyrinth of rooms—a projection of the memories, fears, and past trauma of whoever enters—Clark finds twisted recreations of his life and a mysterious entity dubbed the "Lifeform."
Following his repeated explorations, a worried Mary attempts to find Clark but is knocked out by him and awoken in a pretty bizarre dining room. Joined by the backrooms' other human-like entities, he explains that they mimic real people and locations, and the two argue about why Clark is really the reason his life is a mess.
He eventually realises he can't stay there, and after untying Mary, his newfound outlook leads to the Lifeform (a twisted version of Clark mashed up with his store's mascot) killing and eating him.
It's worth noting at this point that, throughout the film, Async scientists are shown keeping a watchful eye on the backrooms. As an injured Mary flees and successfully fends off the Lifeform, they arrive, capturing the creature and taking the therapist—who has her own issues, including a schizophrenic mother—through a doorway that leads to Async's headquarters.
A scientist, Phil (Mark Duplass), explains to her that the backrooms are one of the most significant discoveries in human history, though Async is also unsure of what exactly they are. They hope that Mary can explain, and the movie ends with her stuck in their custody, unsure whether she'll ever be allowed to leave.
In the final scene, we return to the backrooms, where there are several poorly copied rooms from Mary's past and an entity that looks a lot like her sitting in a chair. She's deformed and has several eyes, confirming that the Backrooms have made a copy of her, the same way it copied Clark.
Backrooms stops short of fully revealing its biggest secrets and certainly doesn't provide any neat explanations for everything we see. However, with Async hoping to map the backrooms, Mary trapped in their labs, and the mention of portals opening across the entire world, it's not hard to see where a sequel could go. After all, what happens as more people enter them and the worst part of their psyches is copied?
Backrooms is now playing in theaters.