What darkness lies in the dreamy imaginative mind of young girls? The mystery has been a source of inspiration for horror classics from “The Bad Seed” to “The Exorcist,” and it could have been a fascinating theme to explore in the internet-inspired “Slender Man,” written by David Birke and directed by Sylvain White. Unfortunately, this profoundly not-scary horror film completely misses the mark about what makes its subject matter interesting.

If you’ve heard of the internet phenomenon that is Slender Man, it’s likely from a 2014 attempted murder in Waukesha, Wis., where two 12-year-old girls stabbed a friend 19 times and left her for dead. They told police they were acting as proxies for Slender Man, a character they discovered on a website hosting “creepypasta” ghost stories copied and pasted from the internet.

The true crime story was covered in the excellent documentary “Beware the Slenderman,” but the horror adaptation takes a different tack, taking the character literally, as a malevolent force that can be summoned with a ritual that’s “The Ring” by way of a slumber party game.

A group of small-town, lightly goth teenage girls stumble upon Slender Man during a sleepover — Hallie (Julia Goldani Telles) jokingly tells her mother “we’re going to drink vodka and meet guys on the internet,” in the film’s only winking line of dialogue, and that they do. Soon, the friend group is disappearing, and the girls are beset with heinous visions of Slender Man, a tall, faceless man in a black bespoke suit. What does he want? Them. What’s he going to do once he gets them? We’re not exactly sure.

That’s the problem with a Slender Man horror movie — there are no rules, because there is no mythology. The character was invented in a 2009 Photoshop contest by Eric Knudsen, aka Victor Surge, and crowdsourced stories and viral videos hunting for Slender Man have made up the lore. In “Slender Man,” we don’t know anything about him, or what to fear, and the film doesn’t fill that in. All we know is he likes lurking in the woods and strangling young girls with his long, treelike fingers — and yet somehow he also has the ability to FaceTime menacingly.

Birke’s script is plainly straightforward, a simple supernatural chase story. It doesn’t plumb the depths of what might make Slender Man scary, so “Slender Man” isn’t scary at all. There’s no tension and no suspense, because even when the girls are panicked and screaming as Slender Man attacks them in their imaginations, we’re constantly yanked back to a fairly normal reality.

What’s truly terrifying (and what “Beware the Slenderman” deftly illustrates) is the vulnerability of young susceptible minds exposed to all manner of folklore and fake news online, how suggestion can inspire outlandish fantasies and even violent action. “Slender Man” brushes up against that theme with the increasingly hysterical Wren (Joey King), but falls back again and again on the ghost story, staying firmly in the realm of paranormal horror, where unfortunately, the scares are increasingly svelte.

“Slender Man” has a moody, atmospheric and often abstract aesthetic. Shot by Luca Del Puppo, with a starkly desaturated palate of browns and grays, it’s sometimes so dark that it’s hard to see. That murky mysteriousness could lend well to the mystery of Slender Man, but instead, White puts Slender front and center. Is Slender Man real, or the figment of an overactive, internet-obsessed morbid imagination? The film tries to have it both ways, and it ends up with nothing at all.