The film takes place in England in 1865. Having been grotesquely disfigured in an African voodoo ceremony for a transgression against the native populace, Sir Edward Markham (Alister Williamson) is kept locked in his room by his guilt-ridden brother, Julian (Vincent Price). Tiring of his captivity, Sir Edward plots to escape by faking his death. With the help of the crooked family lawyer, Trench (Peter Arne), they hire witchdoctor N'Galo (Harry Baird) to concoct a drug to put Sir Edward into a deathlike trance. Before Trench has time to act, Julian finds his "dead" brother and puts him in a coffin (the title's "oblong box"). Embarrassed by his brother's appearance, Julian asks Trench to find a proxy body for Sir Edward's lying in state. Trench and N'Galo murder landlord Tom Hacket (Maxwell Shaw) and offer his corpse to Julian. After the wake, Trench and his young companion Norton (Carl Rigg), dispose of Hacket's body in a nearby river, while Julian has Sir Edward buried. Now free of his brother, Julian marries his young fiancée, Elizabeth (Hilary Dwyer), while Trench, Norton and N'Galo go their separate ways.
Sir Edward is left buried alive until he is dug up by graverobbers and delivered to Dr. Newhartt (Christopher Lee). Newhartt opens the coffin and is confronted by the resurrected Sir Edward. With his first-hand knowledge of Newhartt's illegal activities, Sir Edward blackmails the doctor into sheltering him. Sir Edward then conceals his face behind a crimson hood and embarks on a vengeful killing spree.
Norton is first on Sir Edward's list and has his throat slit. In between killings, Sir Edward finds time to romance Newhartt's maid Sally (Sally Geeson), but when Newhartt finds out about their affair, he discharges Sally and she goes to work for Julian. While searching for Trench, he is sidetracked by a couple of drunks who drag him into a nearby tavern. Here he ends up with prostitute Heidi (Uta Levka), who tries to roll him for his money, only to be killed by Sir Edward's knife. The police get involved, and the hunt is on for a killer in a crimson hood.
Meanwhile, Julian has become suspicious about the body which Trench supplied to him, after his friend Kemp (Rupert Davies) finds it washed up on a riverbank. Julian confronts Trench, who tells him the truth about Sir Edward's "death". Soon after, Trench is dispatched by Sir Edward, but not before he tells him the whereabouts of N'Galo. Hoping he will cure him of his disfigurement, Sir Edward asks N'Galo for his help. Here Sir Edward learns the truth about his time in Africa: in a case of mistaken identity he was punished for his brother's crime of killing an African child. N'Galo fails to cure Sir Edward, and they fight; N'Galo stabs Sir Edward and is rewarded with a face full of hot tar. Sir Edward returns to Newhartt's home, where Newhartt tends to his wound. Mistrusting Newhartt's medical treatment, Sir Edward fatally wounds him and sets off to confront his brother.
Back at the Markham ancestral home, Julian learns the whereabouts of his brother from Sally and leaves for Dr. Newhartt's home, only to find him near death. Meanwhile, Sir Edward arrives back home to find Sally, who is repulsed by her former lover's killing. Sir Edward drags her out onto the grounds of the house, pleading for her love. Julian arrives back home and gives chase with a shotgun. Out on the grounds, Sally snatches Sir Edward's hood from him and his deformed face is revealed for the first time. Julian catches up and Sir Edward confronts him about his crime. As Sir Edward lurches forward, Julian shoots him. Cradling the dying Sir Edward in his arms, Julian is bitten by him.
Once again in his "oblong box", Sir Edward is resurrected by a vengeful N'Galo, but this time he is six feet under with no hope of escape. While back at the Markham mansion, Julian is occupying his brother's old room, where he is beginning to show the first signs of the voodoo curse. His wife Elizabeth learns this when she tells him about his brother's room: Julian then says, "It is now my room". She is flushed with fear of his disfigurment and the film (with credits) closes with a close up of her widened eyes.Vincent Price - Julian
Christopher Lee - Dr. Newhartt
Rupert Davies - Kemp
Uta Levka - Heidi
Sally Geeson - Sally
Alister Williamson - Edward
Peter Arne - Trench
Hilary Dwyer - Elizabeth
Maxwell Shaw - Hackett
Carl Rigg - Norton
Harry Baird - N'Galo
Godfrey James - Weller
John Barrie - Franklin
Ivor Dean - Hawthorne
Price, Davies and Dwyer had recently appeared in Witchfinder General, under the direction of Michael Reeves, and on 18 November 1968, the four also began work on the Oblong Box. The original script had the Markham brothers as twins, both played by Vincent Price. However, Reeves died during pre-production, so Hessler stepped in and made a number of substantial changes. Another man who got called in was Christopher Wicking: he was called in after the sudden death of writer Lawrence Huntington as shooting began.
With the help of Christopher Wicking, he reworked the screenplay to incorporate the theme of imperial exploitation of native peoples in Africa. This theme gave the film a "pro-black" appearance that would later cause it to be banned in Texas.
The leading role of the film was given to character actor Alister Williamson, his first. Although he has the largest amount of screen time, more than either Price or Lee, his real voice is never heard (it was redubbed by another actor) and his face is covered for the majority of the film. His makeup was done by Jimmy Evans, whose other credits include Captain Kronos – Vampire Hunter (1972) and Hessler's Scream and Scream Again (1970).
Hessler says AIP insisted he use Hilary Dwyer.:
I don't know what the situation was, but they liked her and they kept pushing you to use certain actors. I guess the management must have thought she was star material or something like that.
According to Hessler, the film was very successful and ushered in a series of follow up horror movies for AIP including Scream and Scream Again and Cry of the Banshee.
Rotten Tomatoes, a review aggregator, reports that 57% of seven surveyed critics gave the film a positive review; the average rating was 4.6/10. A. H. Weiler of The New York Times wrote, "The British and American producers, who have been mining Edgar Allan Poe's seemingly inexhaustible literary lode like mad, now have unearthed The Oblong Box to illustrate once again that horror can be made to be quaint, laughable and unconvincing at modest prices." Variety wrote, "Price as usual overacts, but it is an art here to fit the mood and piece and as usual Price is good in his part."