Upon arriving at her rental property, “Tess” – portrayed by Georgina Campbell, who occasionally leans into excessive acting – finds that "Keith" (Bill Skarsgård) has already checked in due to a booking error. After an awkward initial meeting and a glass of red wine, they decide to share the accommodation for the night. The following morning, while searching for toilet paper, she stumbles upon a concealed room in the basement that suggests prior illegal activity. When her temporary roommate returns, he investigates, and it’s then revealed there's an underground tunnel system populated by... While I appreciate Zach Cregger’s attempt to create intense, confined horror, I wasn’t entirely sold on the believability of the story from the outset. It seems unlikely that anyone would find something like what she does and then proceed to explore it independently without taking basic safety measures. The tension and danger feel manufactured, overriding any instinctive sense of caution one might expect. Eventually, “AJ” (Justin Long) is revealed as the house’s owner and arrives to find their belongings scattered about, joining in the search within the tunnels. The story gains momentum a bit at this point, with escape becoming the priority against an adversary who makes "Gollum" seem mild-mannered. It proved to be fairly entertaining, I did find aspects of it enjoyable, but the realism of certain events was problematic for me; perhaps because I'm easily frightened, I never fully bought into what was happening. Still, it’s a diverting film and ranks among the better offerings in this frequently disappointing category.

Source on TMDB