Spenser — an ex-cop better known for making trouble than solving it — just got out of prison and is leaving Boston for good. But first he gets roped into helping his old boxing coach and mentor Henry, with a promising amateur. That's Hawk, a brash, no-nonsense MMA fighter convinced he'll be a tougher opponent than Spenser ever was. When two of Spenser's former colleagues turn up murdered, he recruits Hawk and his foul-mouthed ex-girlfriend Cissy, to help him investigate and bring the culprits to justice.
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.