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 (actor)) 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.