This is one spooky film, yet there are no horror or supernatural elements per se, even though it feels like there are. Probably my favorite film from my childhood along with Disney's "The Scarecrow of Romney Marsh". It was years later before I discovered that they had the same source material and they were released the same year. Disney sued Hammer over rights to the character of Dr. Syn, so Hammer changed the main characters name to Dr. Blyss, and the Scarecrow became a supporting character instead of the lead.
I have to disagree with the reviewer who called this a confusing mess, as the storyline is pretty basic and quite clear cut. The movie is filled with such atmosphere that you can practically get a cough from the constant fog. The highlight of the film is the Marsh Phantoms, a group of smugglers who dress up as skeleton and ride skeleton horses. It is a very eerie sight to see.
Special mention should be made of the incredibly beautiful Yvonne Romaine. She only made a handful of films yet she may possibly be the most beautiful woman who ever appeared on screen.
I have heard that the reason this film is not available is that Universal (I think) has the rights to it and refuses to do anything with it. Sad for all of us.