If I recall correctly (and I may not), the original story is (like so many of Derleth's works) an unsuccessful and apparently unconscious rewrite of a pre-existing work by Lovecraft. In this case,"The Shuttered Room" is essentially the same story as "The Lurking Fear," but the original was far from good, and the rewrite is that much worse. As I said, I may not be remembering that correctly, but at any rate, Derleth's story is non-essential imitation.
Not that it matters, really. The film was released in 1967, and was made in the U.K. despite being set in New England, which explains why Oliver Reed keeps referring to his aunt Agatha as "Agather." It was directed by David Greene (Roots, Godspell) who did a lot of TV work, most of it better than this. It's actually very similar to the A.I.P. production of H.P.L.'s "The Dunwich Horror" which would come out a couple of years later, but it's much more boring, and much more obviously made on the wrong side of the pond.
The plot of the movie does not follow that of the short story, or if it does, it does not follow it well. The movie opens eighteenish years ago, in the middle of a dark night while the Whately family is asleep. Something is in the attic, and it clomps its way down the stairs to savage first a sleeping child, and then her parents. We see everything from the monster's point of view, so we have no idea what it is. Then the credits roll, and we skip ahead to the present.
The little girl, Susannah Whately (Carol Lynley), has moved to New York, grown up, and married Mike Kelton (Gig Young (who?)), a well-to-do magazine editor who must be twice her age or more. Mike and Susannah have come to Dunwich Island to visit Susannah's old stomping grounds and see if the Whately mill can be salvaged. Problem: half the people on the island are Whatelys, and none are fond of outsiders. In particular, Susannah's cousin, Ethan (Oliver Reed), is a bully who believes that the mill belongs to him. We also meet dear old aunt Agatha, who whispers curious prophecies and tells us of how Susannah was sent to live in New York after her parents were suddently struck by lightning.
This being Derleth, there's a supernatural element, but it's weak and the poor adaptation takes forever to get around to it. Mostly what we get is Mike and Susannah being harassed by Ethan and his gang of hoods who cover their British accents by imitating the Kennedys. By the end of the movie you've forgotten that there's a monster in the attic, and the monster turns out to be Susannah's twin sister, Sarah, whose only defect appears to be that she's been kept in an attic all her life (oh, incidentally, watch the preceeding paragraph for spoilers).
I was bored with the movie, and part of that has to do with the fact that Derleth makes me angry (did I mention that he was a bastard?), but for the most part it's a horror movie that substitutes fisticuffs for the scary bits. Speaking of which, Mike turns out to be some kinda martial arts master, which I suppose is intended to give a certain depth to the character, but it comes off as silly.
I watched this one just to scratch it off my list, even though I knew it wasn't well liked. If this is on your list, I recommend that you scratch it off without even bothering. It is worse than I'd imagined in that it's boring, and that's the most unforgivable sin a movie can commit.
I couldn't find a trailer for this one, but I did find a trailer for The Lurking Fear, which is a piece of crap and still better than this dreck. No, wait, here's a trailer for Bleeders, which is a better version of "The Lurking Fear."
I think I was going somewhere with all of this, but I've derailed my train of thought. Just as well, really.