A theoretical model has been developed to describe the generation of Sum Frequency (SF) light from a monolayer film adsorbed to the surface of a dielectric and metal composite substrate. This type of substrate provides enhanced SF signals from the monolayer without modifying the intrinsic (hydrophilic) properties of the dielectric surface. The fundamental equations of both resonant and nonresonant SF generation have been extended from a single interface to encompass the two displaced sources, the air/dielectric interface at which the monolayer film is located, and the dielectric/metal interface. The model describes the coherent addition of three separate SF signals which coherently interfere with each other, thereby affecting the line-shape observed in the net SF spectrum. Representative calculations have been made for a highly ordered monolayer of an aliphatic hydrocarbon molecule, adsorbed on a dielectric with the optical properties of mica, which is itself in contact with a metal modeled to resemble gold. The SF spectral line-shapes are predicted as a function of the thickness of the dielectric material for the methyl terminal groups of the hydrocarbon chain. The model can be applied to any dielectric/metal composite by modifying the optical properties appropriately.