Despite its ominously evocative title, HBO’s Beware the Slenderman is not a horror movie. It’s contemporary tragedy. And it wears its sadness on its sleeve.

The documentary, directed by Oscar-nominated and Peabody-winning Irene Taylor Brodsky, explores an attempted murder that took place in Waukesha, Wis., in 2014. Two 12-year old girls with no prior behavioral issues, Anissa Weier and Morgan Geyser, became obsessed with an online character called Slenderman. Reinforcing each other’s delusions, they became convinced that they needed to kill someone in order to both become Slenderman’s servants and protect their families from him. After leading a mutual friend into the woods, they stabbed her 19 times before setting off on foot for Nicolet National Forest, where they believed Slenderman’s mansion to be located. The two were eventually picked up by the highway patrol. The victim survived the attack after dragging herself to a ditch on the side of the road and calling to a cyclist for help.

The story is terrifying for a number of reasons, some more obvious than others. For one, it took place in a wholesome suburban town. In 2012, Money magazine ranked Waukesha as one of the best 100 places to live in America. The following year, America’s Promise Alliance ranked Waukesha as one of the best 100 communities for young people in America. And while it might technically be considered a suburb of Milwaukee, Waukesha appears exceedingly bucolic and is known historically for the purity of its spring water.

By all accounts, it seems like the perfect place to raise a child. Which is relevant, of course, because of another grisly aspect of the crime: it was committed by kids, and not the kind of troublemakers everyone already has their eye on. These were soft-spoken and creative, if somewhat socially excluded, girls. No one saw it coming. That the inner world of these girls was so well-hidden from their parents, and contrasted so dramatically from their physical surroundings, was facilitated entirely by the internet. And perhaps the most lurid aspect of the crime was that these girls were willing to kill on behalf of an internet meme.

Slenderman has been around for a while, at least in internet time. Created by Eric Knudson in 2009 as a character in online “creepypasta” forums (more on that later), Slenderman is usually depicted as a white, faceless humanoid in a suit who can teleport and enter dreams. He’s usually found around woods or abandoned buildings, and seems to simultaneously prey on and protect children, especially abused ones. The literary critic Jack Zipes, one of the experts interviewed in the documentary, links Slenderman to classical German fairy tales and their ability to express ancient and collective anxieties. Seen in this light, Slenderman is a sort of complicated Pied Piper who both protects children from the harshness of reality and leads them deeper into his own clandestine world. Some Slenderman myths even give him a history of abuse himself. It’s an interesting origin story for a mythical character who simultaneously comforts and horrifies children.

It’s a crowd-sourced myth. The “creepypasta” online forums that it sprang from are places to spread horror stories. The tales are copied and pasted from forum to forum and sometimes slightly altered, so that the myth gradually evolves. There can’t be a biography of Slenderman, only a prosopography of his devotees. And while the Slenderman phenomenon may well be a “virus of the mind,” as Richard Dawkins calls it in the documentary, it’s also a self-referential cult of belief.

Slenderman was created, and continues to be recreated, under the noses of adults. And for all of the metaphors that we use to describe it—webs, highways, etc.—the internet cultivates as much tribalism as it does connection. Online “communities” disperse into innumerable micro-experiences. The theorist Dominic Pettman calls this aspect of our contemporary media environment “hypermodulation,” and it allows children to form their own secret cultures, with parallel histories, symbolic referents, and memes. This dynamic was probably always at work in the social lives of children, but never on this scale, with this sort of connectivity, and at this speed.

But hypermodulation alone, however unprecedented, can’t account for these particular girls trying to kill their friend. There are, after all, millions of American children on the internet and not all of them are attempting to commit murder. This case was special. It might pull back the sheet on a collective anxiety, but it’s horrific by virtue of its uniqueness.

Footage of the defendant’s parents and grandparents, interwoven with interviews and statements from experts, forms the film’s double-helix narrative. It’s is a mystery that begs to be solved.

The parents, moving in their sad confusion, are as puzzled as we are. But their question isn’t how this could ever happen, or what sociological lense we should use to analyse the internet, but how could Morgan do this? How could Anissa do this? The girls are portrayed as two sad but feckless outsiders, bullied (like Slenderman?) and rejected from larger social groups, who found a sort of echo chamber of alienation in each other. Sometimes peer pressure is a good thing if it keeps you from entertaining violent delusions. But that was missing.

The real horror of the documentary is the powerlessness of adults to prevent the violence. The parents are haunted and tearful. Morgan’s father, who suffers from schizophrenia, feels implicated in passing his disease to his daughter without noticing its manifestations in her. A former teacher openly weeps, wishing he had done more but wondering what that could have been. Anissa’s father is understandably upset about his son getting mandatory iPads at school. The film is a grim reminder of the limits of parental power and that evil almost never has a satisfactory explanation.

By the end of the documentary, the girls have become internet memes themselves. There’s fan art and fan fiction based on them, and they almost seem to have been recycled back into the myth-making engine of the internet. But the most interesting reactions come from the adult world. The woods where the girls stabbed their victim is razed—the trees uprooted and the soil flattened. A judge decides that Anissa and Morgan will stand trial as adults.

And these seem like predictable, if pitiful, reactions. Retroactively defining our children as adults exonerates us from our occasional powerlessness to protect them, even from themselves.

Scott Beauchamp is a veteran and writer based in Portland, Maine.