This investigation concerns a TEM wave obliquely incident onto the open end of a parallel plate wave guide with a truncated upper plate. The lower plate as well as the dielectric slab between the two plates is assumed to extend out indefinitely in order to support the propagation of at least one surface wave mode. Reflection from the wave guide opening is determined as a function of incident angle via a dual Wiener-Hopf formulation. It is found that total reflection, i.e., a reflection coefficient with magnitude equal to unity, is possible for incident angles greater than the critical angle associated with the lowest-order surface wave mode of the grounded dielectric slab. The field external to the parallel plate wave guide in this case becomes completely evanescent in the cross-sectional plane perpendicular to the plane of incidence, which is determined by the propagation direction of the incident wave and the edge of the upper plate. It is shown that the phase of the reflection coefficient thus obtained can be used to construct the modal equation for the fundamental mode(s) of a wide microstrip.