HBO’s “Beware the Slenderman,” premiering Monday (Jan. 23), is a documentary about the dangers of the online world, common teenage struggles with inclusion and belonging, mental illness — and a growing parental divide in the Internet age.

It also illustrates and warns about the Slenderman, born in 2009. Since his creepypasta inception, the tall, ominous boogeyman — who lives in the woods and prays on innocent children — has grown from obscurity to legend. You might call Slenderman the first modern folk tale: Created by one of the communities online whose joking attempts to coin mythology occasionally strike hidden gold.

But while “Beware the Slenderman” explores the viral sensation of the Slender meme — which continues to thrive eight years later — the documentary is mainly a true-crime tale about two antisocial friends who find solace in each other, and online. It’s this disconnection from their peers and parents that ultimately leads them down a very dark road.

On May 31, 2014, 12-year-old friends Morgan Geyser and Anissa Weier were apprehended by law enforcement after stabbing a fellow classmate, Payton Leutner,  19 times — with the intention of both murdering the girl, and appeasing and becoming proxies for Slenderman. The protection of the ominous creature, in their minds, meant their families would be safe — and they’d be granted the privilege of living in the Slender Mansion, somewhere in the woods.

beware the slenderman hbo 11 Beware the Slenderman is jarring, heart wrenching & uncomfortably real

It’s a story we’ve heard before. Peter Jackson’s “Heavenly Creatures” was also based on a true story. The real monster, of course, isn’t a faceless, tentacled boogeyman — it’s the very real issues, medical and social, to which children can be prey, without oversight. It’s iffy to blame the parents, of course, in cases like this — but any child slipping through the cracks is ultimately the responsibility of every adult around her… And that does include the parents. The “Heavenly Creatures” story, and hundreds like it, have been happening since long before the internet’s inception, and vaguely blaming technology for any of it is equally dubious.

The movie makes a case that multiple culprits are to blame for the attempted murder that has kept Geyser and Weier in prison. There is significance to the way the internet binds us all together in realtime: A hive-mind that turns an innocent (if uncanny) work of fiction into viral Internet lore overnight is a legitimate change in our lives from what has gone before. But the violence, and the fear, and the unspoken history behind it, is nothing new: As long as there have been parents and children, there have been these sad folies à deux.

RELATED: True-life horrors of an Internet meme — HBO’s ‘Slenderman’ doc is coming

In America especially, the stigma of mental illness creates its own conditions: In one of the more emotionally charged moments of the film, one of the girl’s fathers reveals he is a high-functioning schizophrenic. Still, the parents got no real red flags from their child’s behavior as tinkering on an iPad and perusing YouTube videos is nowhere outside of normal behavior.

It’s the heartbreak in their informative tales of their children’s upbringing — flashbacks are presented in the guise of photos and home movies — that further bring home the notion that nothing out of the ordinary had transpired to inspire such murderous behavior.

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Still, without ever showing the girls’ faces in the courtroom scenes, we are given ample tales of the grandiose that finds schizophrenia — and a lack of acceptance by their peers — behind the bloody act. It’s clear through the testimonies presented here that neither girl had bad blood towards their victim, or evil intentions — which is what makes it so frightening. If we can’t find a boogeyman in a story about the boogeyman, the inescapable conclusion is that there is no “them,” no population of markedly different villains and demons: There’s just us.

“Beware the Slenderman” doesn’t resolve, there is no final bar to the song. The trial of the two girls is still ongoing, those three families will never really find peace.

slenderman doc Beware the Slenderman is jarring, heart wrenching & uncomfortably real

What the film does do is explore humanity’s fascination with the boogeyman — the Erlking, the Pied Piper of Hamlin, Freddy Krueger — and provides this horrific real-life tale as a message to parents: Remember what you signed on for when you had kids, be present, be aware — and don’t let your own life loom larger than their small and growing ones. It’s not as simple as surveillance nor as easy as cutting screen time: It’s about knowing who your children are, and the children around you. They’re the adults we are tasked with helping to create.

“Beware the Slenderman” premieres Monday, Jan. 23 at 10 p.m. ET/PT on HBO.

Posted by:Aaron Pruner

When he was a child, Aaron memorized the entire television lineup, just for fun. He once played Charlize Theron’s boyfriend in a Japanese car commercial. Aaron’s a lover of burritos and a hater of clowns. TV words to live by: "Strippers do nothing for me, but I will take a free breakfast buffet any time, any place."