Holly (Julia Goldani Telles) and other girls went to a sleepover at her good friend Katie's (Annalise Basso). During the chat, they learned that several other boys in the same grade were plotting to summon the Slender Man. The Slender Man is a horror legend circulating among the people. It is said that he has long arms and legs, and his face cannot be seen clearly. He specializes in luring and kidnapping children. The brave and curious girls are fearless and found a video on the Internet to summon the Slender Man to make a bold attempt. That night was peaceful, but a week later, Katie mysteriously disappeared during an extracurricular teaching activity. Afterwards, from Katie's father's crying, Holly vaguely felt that her friend's disappearance was related to the summoning ritual they had performed. They entered Katie's room and found that her friend had watched many videos about the Slender Man before she disappeared when they checked the computer records. Maybe the legendary monster really exists...