The scattering of an electromagnetic plane wave by a metallic structure consisting of two parallel metallic half-planes truncated by a plane perpendicular to their edges is considered. The incident field is an arbitrarily polarized plane wave and may be incident from any direction above the truncating plane. The boundary value problem is solved analytically with an exact solution given in the integral form, as well as an asymptotic solution which is valid far from the opening of the parallel plates.