Although credited to Corman, parts of the film were shot by Francis Ford Coppola, Monte Hellman, Jack Hill, and Jack Nicholson. Corman shot footage of Karloff and other actors walking across the sets and downstairs with the belief that he would be able to make sense of them later. In the next three days Coppola, Helman and Hill all tried to do something. Nicholson, who was keen to get directing experience himself, also took a turn behind the camera.
In the early 1990s, actor Dick Miller, who plays Karloff's major domo, was hired to shoot new scenes to use as a framing sequence for an overseas version of The Terror. Under this scheme, the main action of the film is presented in flashback. Today, the film is in the public domain since there is no copyright notice in the credits for the film.
Leftover sets from other AIP films were used when shooting the film, notably those from The Haunted Palace, a Vincent Price horror film made the same year. The tree against which Sandra Knight expires was the same one Price was tied to and burned in Palace.
Clips from the film were used years later in the Peter Bogdanovich movie Targets (1968), also starring Karloff. In 2010, the film was featured in the second episode of the revived, syndicated TV series, Elvira's Movie Macabre.
Set in 1806, the film tells the story of a lost French soldier named Andre Duvalier (Jack Nicholson) saved by a strange young woman named Helene (Sandra Knight). She looks like Ilsa, the baron's (Boris Karloff) wife, who died 20 years before.
Andre begins an investigation to uncover who the woman really is, and stumbles upon a hidden secret of the Baron: he had found Ilsa sleeping with another man named Eric, and in his rage the Baron killed the two of them. Or so he explained.
All the while, the phantom of Ilsa remained under the control of a peasant witch (Dorothy Neumann), who has commanded the ghost to torment the Baron for the previous two years. Over the course of the film, Ilsa's ghost beseeches the Baron to kill himself, so they could be together. After much hesitation, the Baron decides to do so, perhaps to atone for his crimes.
During the climactic scenes, Andre, as well as the Baron's butler Stefan (Dick Miller), try to stop him, eventually forcing the witch into compliance. Here it is revealed that the witch Katrina is in fact the mother of Eric, who was allegedly killed by the Baron twenty years before, and that is why she has tried to make him commit suicide and damn his soul to hell in the process. In a stunning revelation, Stefan reveals that Eric never died, that it was the Baron who was killed. Eric then took the Baron's place, living his life until he deluded himself into thinking he was the Baron.
Katrina, realizing her folly only too late, goes with the two men to stop Eric from flooding the castle crypt and killing himself. However, she is unable to go into the mausoleum, being a witch and therefore of evil association, and ends up being struck by lightning and burning to death outside the gate.
In the climax of the film, Ilsa's ghost attempts to kill Eric while the crypt floods, and Stefan joins the struggle. However, by the time Andre gains access to the crypt, it is already flooding and crumbling, and is only able to carry Helene's body away. The two share a touching moment together outside, only to have Helene begin to rapidly decompose and melt. Katrina's familiar hawk flies away as Helene turns to nothing, and there the film ends.