Whately makes a deal with a malevolent deity to offer his daughter for bearing an offspring, Wilbur, who aims to bring back the Old Ones.

In the rural town of Dunwich, the evil "wizard" Whately makes a deal with a malevolent deity, receiving a lifetime of riches in exchange for offering up his own daughter to bear the offspring of this being. That offspring, Wilbur, is a half-human, half-deity whose purpose is to bring about the destruction of mankind and make way for the return of the Old Ones. But Wilbur also had a twin brother, the titular Dunwich Horror. After Wilbur is killed trying to steal the infamous Necronomicon, this twin brother, who much more closely resembles their father, escapes and wreaks havoc upon the unsuspecting populace of Dunwich.