A classic case of Corman squeezing every penny of value he could get, this was shot largely on leftover sets from other movies, with the director shooting around them as they were torn down; Karloff's scenes were shot in three days, and the entire thing in 11, with various other scenes being directed by Nicholson, Jack Hill and even Francis Ford Coppola, who was credited as an associate producer. Frankly, given all this, it's remarkable that the end product is even coherent. Nicholson plays French soldier Lt. Andre Duvalier, who is separated from his regiment, and ends up seeking shelter at the castle of Baron Victor Frederick Von Leppe (Karloff), after meeting a strange young woman (Knight). Turns out that this may be the ghost of the Baron's young wife, whom he killed years previously, after returning from war to find her in bed with another man. Her spirit now appears to be trying to get him to commit suicide as revenge for her own death. Or maybe not. For there's an old crone living in an abandoned cottage, who appears to have some relationship to the young woman. It's all kinda murky, the storyline shifting from the supernatural to more prosaic, then back again.
I can't honestly say the final twist makes sense: if certainly a surprise, that is likely because it appears out of nowhere, and you can't exactly look back and say, "Oh, that makes sense." Indeed, given the age of the character concerned, and his/her supposed mother, it arguably raises a lot more questions than it answers. However, the film is buoyed by decent performances from most of the cast, and there's an overall atmosphere here which strongly feels like it should be an adaptation of a story by Edgar Allen Poe, even though it isn't. Ronald Stein's score also appears to have strayed in from a far better, bigger-budgeted movie. On the downside, stories that the film started without anything approaching a finished script, would appear to be entirely credible. The narrative here doesn't so much flow, as lurch from one moment to the next, Duvalier roaming the castle and its grounds, seeking an answer to the mystery into which he has unwittingly become embroiled. The outcome will probably have left the character only somewhat satisfied, and much the same will probably be true for the audience.