Description: The Scalar Theory of Everything (STOE) model of photons is extended to model the inertia of mass. The characteristic of the plenum, hods, and their interactions yields the Newtonian characteristics of gravitational mass and inertial mass. The Equivalence Principle is derived from these characteristics rather having to be assumed. The characteristics have been used to derive the STOE photon diffraction model. An experiment has rejected all other diffraction models.