One late spring day in 2014, three girls entered the woods in Waukesha, Wisconsin. Two walked out unharmed. A 911 call made not long after revealed the hazy outline of a vicious attack—one of the girls had been found by the side of the road covered in blood, having crawled there to get help. In the days and weeks that followed, details emerged that were no less disturbing: The three girls, all 12 years old, were best friends. The victim had been stabbed 19 times with a 5-inch blade and had barely survived. After being taken into police custody, the other two girls told interrogators what had happened: They had lured their friend into the woods to kill her so that they could appease someone called Slenderman.

Those who have spent any time on the internet since 2009 and who possess a passing familiarity with memes have probably heard of the fictional subject of HBO’s new documentary, Beware the Slenderman, airing Monday. He’s tall and pale with long limbs and no face. He usually wears a suit. Depending on who’s doing the telling, he abducts children or is otherwise considered an evil spirit; sometimes he’s more of a guardian angel. Most importantly, the two girls at the center of the 2014 stabbing, Anissa Weier and Morgan Geyser, believed he was real—real enough that they insisted the attack on their friend “had to be done.”

Beware the Slenderman, directed by Irene Taylor Brodsky, is a true-crime film that’s less interested in ascertaining guilt (since the girls confessed) than in the psychology and social factors that led to the stabbing in the first place. It deftly examines the rise of the Slenderman myth online, via online horror stories and art known as “creepypastas,” message boards, fan sites, and social media. But it also looks at the tangled intersection of children’s neurological development, the ubiquity of internet access, literary history, adolescent insecurities, and mental illness—as well as the criminal-justice system responsible for punishing the girls. In response to the question, “How could this have happened?” Beware the Slenderman stubbornly refuses to give a simple answer, or any of the expected ones (bad parenting, the internet is evil, children are gullible, children can be monsters). Despite this complexity, Brodsky succeeds at closing the psychological distance between the viewers and a crime that initially feels remote, weird, and unthinkable.

Contrary to the mythology that has built up around Slenderman, which  suggests that he’s been around for centuries, the character was in fact first developed in 2009 for a horror photoshop contest. Since then, he’s amassed a huge online fan base, inspiring people to develop elaborate riffs on his backstory, create fake “footage” of sightings, make themed art, and more. As the documentary shows, it’s this world that captured the imaginations of Geyser and Weier, who have both been incarcerated for the last two years. Though the trial date is still pending, they’re set to be tried as adults for attempted first-degree intentional homicide; if convicted, they could be sentenced to up to 65 years in state prison.

After a quick introduction to the crime, Beware the Slenderman goes back in time—to the childhoods of Geyser and Weier. Not only that, but their childhoods as narrated by their parents. The choice to do so may raise eyebrows; it can be seen as an uncomfortable effort to immediately humanize the perpetrators without offering the perspective of the victim herself, Payton “Bella” Leutner (who has since recovered from her injuries and returned to school). But, in a way, the director’s decision was made for her—Leutner’s family repeatedly declined Brodsky’s requests to interview them and the girl herself.

“Let’s face it, that is an unpopular tack,” Brodsky told me of the choice to spend the bulk of the film’s time on the perpetrators and their families. “I think it’s hard to have sympathy for someone who is capable of such violence and calculates it.” As a result, Brodsky said she felt she had an obligation to go “deep down into the rabbit hole of [Geyser and Weier’s] brains,” to use that extra bandwidth to delve into the psychology of the crime. Though she initially intended to follow the case through to the trial, she eventually realized that doing so would add little to the story: The girls already confessed, and most of the details surrounding the crime had come to light.

Hence, Beware the Slenderman’s unsettlingly close look at Geyser and Weier—unsettling because the film forces viewers to look past the reductive labeling of them as “monsters.” The documentary gets a glimpse of Geyser’s earliest years, cute home videos shot by her mother, before revealing that she had issues with empathy from a young age (her mother recalls Geyser watching Bambi and expressing no sadness when the baby deer’s mom was killed). Later, the film reveals some devastating details about her mental illness—diagnosed as childhood-onset schizophrenia—and her father’s experiences with the same disorder. With Weier, viewers learn about her loneliness, and how she cried constantly while at school, and how she was part of a choir and how her father kept a close watch on her internet use. It’s a picture that’s difficult to stomach alongside the reports that Weier was the one who ordered the killing, with the words “Go ballistic; go crazy,” and that Geyser eventually complied, telling Leutner, “I’m so sorry,” right before stabbing her.

If there’s a main thread to Beware the Slenderman, it’s how blurry the line between reality and fantasy can be for children, and how that line is especially exacerbated by constant internet use and other neurological and social factors. The common fear when it comes to children, especially girls, using the internet is predators—strangers who might manipulate or hurt them. This was Brodsky’s initial assumption when she heard about the attack. “I remember suspecting, erroneously, that someone was trolling them on the internet and had encouraged them to do this—that they were communicating with a real person who was impersonating Slenderman,” Brodsky told me. But as she did more research on the case, she realized: “No one was telling them to commit this act. This was a whole fantastical universe they had created in their head that had its own mythology, its own set of rules.”

The case bears some superficial similarities to another popular creepypasta—a story known as “Candle Cove” (written the same year Slenderman was created). The tale, about an eerie children’s show, was turned into a 2016 Syfy horror show called Channel Zero, whose first season centers on the mind-controlling powers of a piece of children’s entertainment, and adults’ inability to see kids as capable of frightening violence. As kids start using the internet  younger and more frequently (many elementary schools give iPads to every student, for instance), pop culture is likely to see more stories exploring the darker side of how childhood is influenced by media consumption, new technology, and the fuzzy reality-fantasy line.

While Brodsky asks an array of experts to weigh in on the different facets of the case—the literary precedents of Slenderman (the Pied Piper of Hamelin), meme culture, neuropsychology—she doesn’t dig much into the question of whether a Wisconsin judge was right to determine that Weier and Geyser be tried as adults. (State law requires first-degree attempted homicide cases to be tried in adult court if the accused is 10 or older.) “There are only so many experts that can argue this because so much of this boils down to your basic value system as a viewer,” Brodsky explained. “How much are you going to hold a child accountable for this adult playground that we allow them to play in?”  

Despite using footage of courtroom testimony and police interrogation, Brodsky doesn’t try to litigate different aspects of the case (not just the girls’ guilt, but also whether different pieces of evidence should be ruled as inadmissible). As a result, Beware the Slenderman is able to keep its eye on the bigger picture. It’s able to spend time with Weier and Geyser’s parents, and watch them do the difficult work of resuming their daily lives while coping with the fact that their daughters nearly killed someone. It touches on the pain and stigma of mental illness. It presses the importance of monitoring children’s internet use—while acknowledges that such monitoring can only go so far. And the film does all this without fear-mongering or prioritizing sensationalism over its subjects’ humanity. While Beware the Slenderman isn’t an easy watch—and will undoubtedly inspire anxiety and panic in some viewers, particularly parents—it’s a worthy one that helps make sense of a senseless crime.