There is exactly one image in Separation that earns the R on its poster, and William Brent Bell puts it right on the poster: a warped, grinning marionette-thing crouched at the end of a hallway while a small girl stands in the dark and looks up at it. For about three seconds the film is genuinely frightening. Then somebody speaks and the spell breaks — which is, in miniature, the whole problem with a horror movie that designed a nightmare before it wrote a story to put it in.

Released in the spring of 2021, Separation follows Jeff Vahn (Rupert Friend), a stalled comic-book artist whose grim little puppet characters, the Grisly Kin, are the only thing he has ever been good at. He is losing a vicious custody fight for his daughter Jenny (Violet McGraw) to his estranged wife Maggie (Mamie Gummer) and her rich, contemptuous father Rivers (Brian Cox). When Maggie is killed in a hit-and-run, Jeff and Jenny are left alone in a cavernous Brooklyn brownstone — and the drawings on the walls start to move.

For a while you can see the better film trying to claw its way out. Cinematographer Karl Walter Lindenlaub, a veteran of glossy studio spectacle, shoots the brownstone like a haunted dollhouse, all lightless corridors and sickly pools of lamplight, and Bell stages his monster with real relish. The Grisly Kin are performed by the contortionist Troy James, whose limbs bend the wrong way with a dry, insectile horror no CGI could buy; every time one of them unfolds out of the shadows the movie briefly remembers what it is for. Brian Cox, meanwhile, does what Brian Cox does, importing forty years of gravitas into a role that barely earns ten minutes of it.

Everything between the scares, though, is a slow leak. The screenplay by Nick Amadeus and Josh Braun has one clever idea — grief and a custody war made literal as a haunting — and no idea what to do with it for a hundred minutes. Characters explain themselves in flat, functional sentences; the plot stalls whenever it likes, then lurches; a late twist about who is really pulling the strings arrives with neither the logic nor the dread to land it. Bell has been here before — The Boy, The Devil Inside — and the pattern holds: a strong horror image, a script too thin to carry it.

What stings is how close the raw material sits to real horror. In the same era, films like Hereditary and The Babadook mined this exact vein — parenthood, grief and guilt curdling into something monstrous — and drew blood. Separation keeps gesturing at that darkness, at a father who half-wants the thing in the walls to solve his problems, then flinches back to a jump scare and a music sting. For all its R rating it scares like a PG-13, forever pulling its punch at the moment it might have hurt.

Two years on, Separation has settled into the place its reviews assigned it: a footnote in the long boom of American horror it was never quite good enough to join. Critics were unkind, audiences shrugged, and the film’s most durable legacy is a creature design that horror fans still trade screenshots of, detached from the movie entirely — a monster that outlived its own story. That is not nothing. It is also not a recommendation.

See it, if at all, for the Grisly Kin and for Cox refusing to phone it in; skip it for anything resembling a plot, a scare that sticks, or a reason to care about the people it keeps in the dark. Separation is a haunted-house attraction with a superb animatronic and nobody minding the script — proof that in horror a brilliant monster and a broken movie can share one body, and that the monster always loses.