From the liner notes

Dr. Alan Harker rides to the crumbling castle of his former university professor, Dr. Eric Usher, to find the old man caught in the grip of paranoia, his mind eroded by his ongoing attempts to resurrect his comatose daughter Melissa with the blood of abducted women. As he nears inevitable death, Usher enters a twilight world that exists between life and death. There, he is taunted by the blood-drinking ghost of his dead wife, by his faithless housekeeper, and by the various women he has bled to death over the years in the service of his daughter! Spanish horror maverick Jess Franco blends Edgar Allan Poe with echoes of Bram Stoker's Dracula and his own Awful Dr. Orloff in this bizarre tale of horror and obsession!