‘Beware the Slenderman’ is a chilling look at a crime inspired by the Internet

In simpler times, anxious parents worried about what might happen to their children at the park or on the walk home from school. These days, parents fret about the dangers that might lurk behind a closed door in their own home.

“Beware the Slenderman,” airing Monday on HBO, will do little to quell fears over screen time, social media and the influence of technology on young people. The story it tells is — at the risk of sounding like a local news promo — every parent’s worst nightmare.

The documentary, directed by Irene Taylor Brodsky, examines the disturbing case of Morgan Geyser and Anissa Weier, two 12-year-old Wisconsin girls who in 2014 lured another friend into the woods and stabbed her 19 times (she survived). The motive for their crime? Pleasing Slender Man, or Slenderman, a fictional boogeyman popularized through Internet forums, blogs and social media that the girls believed was real.

“Beware the Slenderman” joins a growing list of documentaries that fall under the true-crime banner, but offer something more than lurid sensationalism — “The Jinx,” Making a Murderer,” “O.J. Made in America.” In this case, it’s a deeply unsettling look at childhood mental illness, the blurred line between the virtual and real, and the potency of internet memes.

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“Beware the Slenderman” uses some of the usual source materials — interviews with family members, home movies, police interrogation footage, local news reports — to establish the basic details of the crime itself. Though Geyser was allegedly the one who stabbed the victim — Payton “Bella” Leutner, her best friend since fourth grade — it was Weier who egged her on. “Go ballistic, go crazy,” she reportedly said.

As with other young, violent duos — Leopold and Loeb, Dylan Klebold and Eric Harris — Geyser and Weier complement each other in terrible ways. Weier is socially isolated and bullied, spending seemingly most of her free time online. In one of the documentary’s more affecting sequences, Brodsky recreates one of Weier’s browsing sessions, clicking through ephemera like an “Are you a psychopath?” quiz and a YouTube clip of a woman feeding a mouse to wild cats.

Geyser initially seems the more dominant and calculating of the two, but, we gradually learn, she’s a bright young woman with a strong affinity for fictional worlds. We see snapshots of her dressed as a Vulcan and hear about how she believed in Santa Claus until she was 11.

Both girls have mental health issues that slowly come to light but were not necessarily obvious to their family members, and find refuge in their shared interests. Like millions of other tweens, they love scary stories, and spend hours reading and sharing horror tales on the popular website Creepypasta. They both become particularly fixated with Slender Man, an unusually tall and thin, faceless man who, according to lore, abducts children. In order to save their families from his wrath, they commit to to becoming one of his “proxies.”

Weier’s and Geyser’s parents, who appear at length in the film, are not the neglectful people it’s easy (or maybe just comforting) to assume they are. Weier’s father worries about the influence of technology on his daughter, and is seen trying, unsuccessfully, to pry his son away from the iPad. Geyser’s mother recalls worrying that her daughter never got upset watching films like “Bambi”: “If something bad happened to the main character, she wouldn’t have empathy for them.”

What really sets “Beware the Slenderman” apart is its attempt to place this gruesome crime in a broader cultural context. Brodsky spends a considerable amount of the film’s running time interviewing — via Skype, which seems apt — experts in memes, digital folklore and the Brothers Grimm. They argue that, similar to the Pied Piper or other menacing fictional characters, the Slender Man myth reflects the particular anxieties of its time.

Evolutionary biologist Richard Dawkins even weighs in, describing a meme, even something innocuous like the Ice Bucket Challenge, as “a virus of the mind spread by being listened to or seen.”

Fittingly, Brodsky incorporates (surprisingly non-cheesy) dramatizations of Slender Man into the usual documentary blend of interviews, police interrogations, home movies and local news reports.

The final third or so of “Beware the Slenderman” also raises critical questions about childhood mental illness and the justice system, where high-profile crimes such as this one are often handled with an eye towards retribution rather than rehabilitation. Sadly, despite the very strange and particular circumstances of this case, it’s just like many others.

‘Beware the Slenderman’

Where: HBO

When: 10 p.m. Monday

meredith.blake@latimes.com

Follow me @MeredithBlake