The plot follows a group of wealthy seventh-grade girls who face a night of terror together after a social media game spirals out of control, resulting in murder.

Twelve-year-old Sam is invited to a sleepover at classmate Sofia Cox's mansion in rural Connecticut.

The girls engage in a pretend fashion show, incessantly posting photos of themselves on social media with their smartphones.

Cat arrives, and Alex allows the girls into her walk-in safe where numerous pieces of jewelry and clothing are stored.

Sam goes to search for Cat in the woods and finds Sofia's father's car parked, with blood across the windshield.

She calls her mother; Alex answers, believing it to be her husband, angrily lashing out into the phone about his cheating, and hangs up.

On her phone, she finds photos and videos of her friends' murders posted on a social media game by Cat.

The film ends with a montage of photos of the murders followed by a video uploaded by Cat, which eventually goes viral and she professes her revenge against the girls, saying that she will "be remembered forever."

"[5] Writer-director Subkoff described the film's overarching theme as "cultural narcissism, and how it affects these young women who are killing themselves over being cyberbullied.

"[6] In February 2014, it was announced that Tara Subkoff had written and was going to direct a film starring Chloë Sevigny, Timothy Hutton, Balthazar Getty, Natasha Lyonne, Taryn Manning, Stella Schnabel, Lydia Hearst, and Annabelle Dexter-Jones.

[7] In a later interview with Elle, Subkoff stated that she was inspired to write the film after a discussion she'd had with one of her friends' daughters, who had been cyberbullied.

[9] Prior to shooting, Subkoff spent several weeks doing acting workshops and improvisation exercises with the young cast members in order to prepare the girls for their roles.

[10] Subkoff spent nearly seven months editing the film with her editors Janice Hampton and Catrin Hedstrom.

"[28] Christine N. Ziemba of Paste gave the film a mixed review, writing: "It's a familiar trope—the killer in the woods hunting a bunch of school girls left home alone—and so much less terrifying than kids armed with their phones, social media accounts, and snark and spite.

"[29] Chuck Bowen of Slant Magazine gave the film a two out of five-star rating, summarizing it as "a blunt satire of the dehumanization inherent in social media," adding that Sevigny and Hutton "tear into their material with committed lunacy.

"[30] Jeanette Catsoulis of The New York Times wrote, "for all its bursts of violence and descending-guillotine sound effects, #Horror feels driven more by social commentary than by outright terror.

"[31] Inkoo Kang of TheWrap gave the film a favorable review, writing: "#Horror undeniably succeeds in its main mission: parodying and scrutinizing the ease with which we capture, recycle, reframe, and desensitize ourselves to even the most horrific images.