A systematic boundary-layer approach is for the first time applied to diffraction of a high-frequency plane wave by a contour with a jump of curvature. Assuming that the incident wave is non-tangent, we present a detailed description of the outgoing wavefield within a boundary layer surrounding the point of non-smoothness of the contour. This allows us to describe the wavefield within a transition zone surrounding the limit ray in terms of the parabolic cylinder function D₋₃ which has not been previously encountered in high-frequency diffraction problems.