When Sir Edward Markham (Alister Williamson) returns from Africa with his face horribly disfigured in a voodoo ceremony, his brother Julian (Vincent Price) keeps him locked in the attic. With the help of his lawyer Trench (Peter Arne), African sorcerer N'Galo (Harry Baird), and unscrupulous Dr Newhartt (Christopher Lee), Sir Edward manages to escape and wreak havoc on the town while seeking revenge and the answer to why he was cursed.

The title of the movie comes from a short story, 'The Oblong Box' (1844), by American writer Edgar Allan Poe, but the story has nothing to do with the movie. Poe wrote another short story called "The Premature Burial" (1844) from which the notion of premature burial may have been taken, but this story also has nothing whatsoever to do with the movie. The screenplay for The Oblong Box was written by British screenwriters Lawrence Huntington and Christopher Wicking.

When Julian learns from Sally (Sally Geeson) that the man with a crimson hood is staying with Dr Newhartt, he rushes over, only to find Newhartt lying on the floor, his throat slit. Just before he dies, Newhartt reveals that his attacker was Sir Edward Markham. Julian races back to Markham Manor to find that Edward has abducted Sally and carried her into the woods. Armed with a shotgun, Julian goes in pursuit of Edward and Sally, while Elizabeth (Hilary Heath) goes in pursuit of Julian. Meanwhile, Sally has succeeded in ripping off Edward's mask, revealing his grotesque face for the first time. When Julian catches up with Edward, Edward asks why it was he who the natives disfigured. Julian reveals that it was he (Julian Markham) who killed the little African boy, but the natives mistakenly chose Edward as the Markham on whom to take their revenge. When Edward rushes Julian, Julian shoots him. When Julian goes to help his dying brother, Edward bites him in the hand. A vengeful N'Galo visits Edward's grave and, by using his sorcery, resurrects him to die once more in his coffin. In the final scene, Elizabeth mounts the stairs after hearing Julian summon her with a bell. When she finds Julian looking out of a window, she points out that he's in Edward's room. 'No, Elizabeth,' he replies and turns to face her, revealing that his face is becoming disfigured. 'This is my room.' Elizabeth's eyes widen in horror.

Poe's 'The Oblong Box' is in the public domain and can be accessed on several websites, e.g., here or here.