Jack Hill creates a weird story circling about the strange Meryee family which suffers from some strange disease, causing their members to degenerate into primitive pre-human lifeforms
or as Lon Chaney puts it out "rotting of the brain". Chaney himself, of course, is - in a fine performance - the loyal caretaker who suddenly has to deal with the fact that some relatives are trying to get their hands on the family estate (and heritage).
Besides great cinematography (under the given budget) and the genuine storyline one has to admire the characters, making the viewer think of something like "Addams Family" on Crack.
Classic stuff, with some bits which really are memorable (e.g. Chaney's groan "It's going to be full moon tonight" when the nerd hero and his love interest are discussing horror films in general and "The Wolfman" in special). And, last but not least, there's always Sid Haig...