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.

Robert McCall, who serves an unflinching justice for the exploited and oppressed, embarks on a relentless, globe-trotting quest for vengeance when his former partner is murdered.