Black Christmas is a slasher film of what I like to call the 'speed-demon' variety. Unlike, say, a Friday the 13th film where Jason Vorhees gets a lot of screen-time, often doing a geriatric stroll toward his intended victims in full view of the camera, the speed-demons are not stars: they are disposable villains who must remain in the shadows, anonymous, until they are finally 'revealed' in the third act. The speed-demon is forced to prowl around outside for most of the running time of the film, tending to the cutting of phone lines, slashing tires, leaving calling-cards or other busy work. When it's time to kill, the speed-demon will typically attack with great speed from outside of camera range or burst out of the closet like a Tazmanian devil, slashing a victim into scissor dolls before the camera has time to get a fix on what's going on. Defenestration is also a good tactic for a speed-demon -- one lightning-fast blur of action, and the busty brunette is sailing down toward the pavement, mission accomplished.
Since we usually never find out who the speed-demon is until nearly everyone is dead, the last fifteen minutes of the film must be squared away for a pointless blab-a-thon in which a third-grade sleight, prom date gone wrong or other psycho-forming event is re-hashed in a therapy session at knife-point. Black Christmas mixes this up a bit, providing us with an entire parallel story-line, told in flashbacks throughout the present-day action. It's in those flashbacks that we meet the Lenz clan, who are sort of like a Far Rockaway version of the Texas Chainsaw Massacre family. The mother, played by Karin Konoval, not only keeps her teenage son Billy padlocked in the attic at all times, she also rapes him, gets pregnant, and carries Billy's sister-daughter to term. The link between the flashback action in the 1980s and the present is the family home itself, which has, in the intervening years, become an upscale sorority house populated by the likes of Lacey Chabert and Michelle Trachtenberg.