In a small town in Massachusetts, four high school girls perform a ritual in an attempt to debunk the lore of Slender Man. When one of the girls goes mysteriously missing, they begin to suspect that she is, in fact, his latest victim.

本作は、2004年に行方不明となったハンドルネーム“はすみ”という女性が、匿名掲示板に実際に書き込んだ内容をもとに製作された作品。