Eventually, "Satan" dispatches the boyfriend (or appears to do so), and Dougie's sister and mother convince themselves that the killer in the costume is the boyfriend who has really gotten into the role. "Satan" never speaks, but gets to get a little frisky and creepy with the family before the return of the boyfriend, not nearly as dead as we thought. The killer then reveals his real nature, leading to a series of close-calls, deaths, and attempts at camp-humor, culminating with the killer trying to get revenge on the townspeople with Dougie's mother as a hostage and instrument of the act.
My synopsis is better than the film. Lame story, lame acting, lame special effects... just lame.