Brimstone and Treacle centres around the heavily troubled Bates family. The marriage between Mr. and Mrs. Bates is rocky, and both are suffering under the strain of caring for their mute, handicapped daughter Pattie. When Martin, a mysterious stranger arrives, they welcome him as a surrogate member of the family, especially pleased by his eagerness to help care for Pattie. Martin, however, is not all he appears to be. Potter uses the archetypal scenario of the supernatural visitor to explore the ambiguities of good and evil, as some of Martin's demonic acts have unexpectedly positive effects.