Filmed back-to-back with Plague of the Zombies, and sharing a number of the same cast, notably Ripper and Jacqueline Pearce - you might also find some of the sets a bit familiar if you watched the two (as we did) almost consecutively! Harry Spalding (Barrett) and his wife Valerie (Daniel) head for Cornwall to move into their inheritance, a country cottage bequeathed to them on the sudden and unexpected death of Harry's brother. However, they are not exactly made welcome by the locals, except for the local inn-keeper, Tom (Ripper). They encounter some of the "colourful", shall we say, local characters, such as Mad Peter (John Laurie from Dad's Army) and Dr. Franklyn, a scholar who has returned from the exotic Orient where he was studying their native religions, and now lives in the local manor-house with his daughter, Anna (Pearce). Their strange behaviour - and the death of Mad Peter at the Spaldings' dining-room table - helps set things up for an eventual encounter with the titular creature.
The set-up is probably better than the pay-off, which seems to involve not much more than interminable sequences of various people creeping around darkened corridors, and some strange decisions made by the lead characters, in particular Dr. Franklyn. Until then, the supporting cast really does a good job of setting up the creepy atmosphere, with especial credit to Ripper and Laurie, so much so that you really wonder why the Spaldings don't do what would appear to be the sensible thing, and leave by the first train. There's a very obvious plot-hole, in that the same venom which appears to knock anyone dead on the spot, has much less effect on Harry when he is injected, and there are also some continuity bloopers which even we struggled to ignore. I did like the creature design, which is fairly simple, yet effective, and doesn't look nearly as bad as I thought it might. Despite running out of steam, this is at least an original entry in the Hammer canon, showing they could come up with their own ideas too.