Matthew Holness has written and directed a horror film that his TV alter-ego, Garth Marenghi would hate. And that’s a good thing.

Possum is a haunting psychological horror film starring Sean Harris (Mission Impossible: Rogue Nation, Mission Impossible: Fallout, Prometheus) and Alun Armstrong (Krull, The Mummy Returns, Sleepy Hollow) that powerfully captures the horrific feel of 70s British public information films cautioning children against doing dangerous things, and features an original score by the BBC’s Radiophonic Workshop – responsible for iconic 70s soundtracks for TV shows like Doctor Who.

This classic 70s feel isn’t inspired by, or intended to play on nostalgia, however. This is the world that lives on inside the severely damaged psyche of Harris’ lead character Philip, a disgraced puppeteer who returns to his disgustingly rundown home in Norfolk, England. This childhood home – a dank, moldy, nightmare-inducing series of hallways and bleak, barely furnished rooms – is maintained by his equally disgusting uncle Maurice (Armstrong). At the same time, a local teen boy has disappeared; the same teen that Philip awkwardly tried to chat up about his art on the train into town.

I almost forgot to mention our title character, Possum. Philip carries a leather bag with him everywhere he goes, and inside that bag is the physical manifestation of all of his trauma and psychosis, the disturbing puppet, Possum – a horrific human face mounted on a spider body the size of a small dog. Philip is desperate to dispose of Possum after some unspecified scandal that has cost him his job, but Possum isn’t having it.

Holness’ feature film debut is nothing short of impressive, even if it doesn’t quite stick the landing. Based on his own short story from the 2008 collection, The New Uncanny, Possum draws its inspirations from sources as varied as German silent film, George A. Romero’s Martin, and the sexual abuse case of British entertainer Jimmy Savile. From the opening moments of the film, we don’t know whether to sympathize with or to fear Philip, in strong part due to the immersive performance by Sean Harris.

Harris’ entire body is constantly in use to portray Philip’s internal struggles. He shambles like a marionette, his arms clenched inward, his gaze always downward, his mouth in a perpetual scowl. Even at rest he is never relaxed. He is always on edge. And just beyond that edge, Possum is always present. Try as he might, whether it’s by drowning, burning, or abandonment, Possum always returns to his place, hanging on Philip’s bedroom wall – or even more horrificly, lying in bed next to him when he wakes up from surreal nightmares involving thick black smoke, bunches of balloons, and/or an unnerving patch of woods where he repeatedly finds himself placing Possum’s bag in a confluence of trees that mimic the legs of spiders.

Armstrong is equally effective as the abusive Uncle Maurice, constantly hinting at secrets while ridiculing and dominating Philip in subtle ways that imply a grotesque hidden past. Once the news breaks that a boy has disappeared, it’s equally possible that Philip, Maurice, or Possum is responsible. Or maybe there’s nobody in the story but Philip. In fact, even after the events in the finale play out, I’m still not quite sure any of the characters in this film but Philip and the abducted boy are real.

For a first film, Holness displays a mastery of tone and mood that is dark and disturbing in a way few experienced filmmakers manage. Possum is a film that features no gore, no overt adult language, no nudity or explicit elements, and could ostensibly play on TV on a Sunday afternoon, subsequently fucking up the childhoods of an entire nation of children trapped indoors on a rainy day. As such, it’s a goddamn masterpiece of horror, despite an ending that is a bit predictable and underwhelming – unless, of course, it’s all in Philip’s head.

And Philip’s head is not a place anybody would want to be.

Possum will be released in theaters and on digital platforms November 2, 2018.