A bunch of teens decide to go up to The Old Dark House on the night of the full moon. As they mount the creaking stairs up to the front porch the nerdy guy of the group says, “I don't think this is a very good idea guys.”
Of course it's not.
What the teens don't know … but what everybody in the theater audience knows … is that somewhere in The Old Dark House is:
An escaped lunatic from a nearby insane asylum who has returned to the house where he committed terrible unspeakable murders. It was, in fact, This Very Night 20 years ago when he took an axe and chopped up his entire family.
Or …An Alien needs to extract fresh pituitary glands from human beings between the ages of 18-24. It needs to do this once every 20 years in order to continue to masquerade as a human and This Very Night is its last chance.
The house was once inhabited by satanists who performed arcane rituals to open a Portal to Hell in the cellar. A blood sacrifice is needed to keep the gates of hell closed for another 20 years and … you guessed it … The Door opens This Very Night.
Hidden in the cellar are the rotting bodies of the victims of the current slasher killer who has been terrorizing the town. Unbeknownst to the teens is one of their own group is the killer. And is probably related to the lunatic currently imprisoned in a nearby insane asylum for hacking up his family This Very Night 20 years ago.
Regardless … it's past curfew … and the band of teens enter The Dark Old House.