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A family discover a tall, faceless entity that could be linked to the recent cases of missing children after their daughter was kidnapped.
The film begins with three student filmmakers, Jay, Alex, and Amy walking through the woods at night filming a documentary about a tall, faceless creature called the Slender man. During the walk, Jay notices a page nailed on a tree that reads 'Help Me'. Alex tells to viewer that the pages were left behind by victims of Slender man before they died. Then they find another page that reads 'Can't Run' and another that reads 'Follows'. Along the way, something goes horribly wrong. Alex's leg is grabbed by some black tentacle and it dragged him away leaving Jay and Amy in fright. They find another page that reads 'Always Watches No Eyes' and then Jay gets dragged away leaving the camera on the ground until, a terrified Amy picks it up and apologies to her family knowing that she's next. Then she finds another page that reads 'Don't Look Or It Takes You'. Static starts picking up and Amy hears an intimidating voice saying her name. Ignoring the note, she turns around and see's the Slender man. She screams before it attacked her and the camera shuts off.
The next scene takes place in a suburban neighborhood where the Benson family live. Eric, his wife, Julia, their two daughters, 16 year old Alexis and 5 year old Emilia, and their strange grandmother, Doris. After her husband died, Doris asked her son, Eric if she can move in with the family because she doesn't like living alone and ever since she moved in, she hanged Crucifix's in every room of the house. Every time her grand kids asks her why she's being so odd, she replies that its for their own good. That night, Doris walks into Emilia's bedroom, and see's her drawing some pictures. She shows Doris some drawings of a faceless man wearing a black suit which terrifies her.
One morning, the family went to the park. Alexis was told to watch her sister, but she got some texts from her friends and wanted chat with them. So, Emilia went to alone to fly her kite until a strong breeze blew it to a tree. When she tried to get it down, The Slender man appeared and got it down. Her family heard her scream and she disappears. The family started a search party to find Emilia. Doris see's the Slender man her window, and tells her that Alexis is next. This leads Doris to tell her family that the Slender man stole Emilia. And she knows him because she too was a victim of the Slender man.
When she was a little girl, she went hiking with her parents until he chased a butterfly and got lost in the woods, The Slender man then dragged her to his mansion and locked her in the basement. Eventually, he was going to eat her but, she managed to escape, and then her parents found her. But the Slender man continued to haunt her to this day, and as a result, she went crazy and became obsessed with protecting herself and her family from him. So they locked all the doors and boarded all the windows hoping to keep Alexis safe. Little did they know that Slender man has teleportation powers. That night, while everybody was asleep, Slender man teleported into their house, grows tentacles from his back and drags Alexis from her bed with them. Eric and Julia woke up as she screamed, but the Slender man disappeared. Suddenly, a group of ghost children (whom were killed by Slender man) appeared and guided them to an abandoned mansion where he lives. When they arrived, Alexis and Emilia are stand in front of the mansion, brainwashed. Slender man grows a mouth and is about to eat the girls. Until, Doris told him to let them go and take her instead. So, he grabs her and swallows her whole and before disappearing. The brainwashing curse within Alexis and Emilia break and they happily reunited with their parents. One month later, they gather together to mourn the loss of their grandmother who sacrificed herself for her grandchildren.
In a post-credits scene, Slender man targets his next victims.
Lin Shaye as Doris Benson
Patrick Wilson as Eric Benson
Kathryn Hahn as Julia Benson
Mackenzie Foy as Alexis Benson
Maya Le Clark as Sally Benson
Ralph Fiennes as Slender man
Jeremy Irons as Slender man (voice)
Main article: Slender: The Arrival
The production of a sequel was announced.