We theoretically explore the spectral behavior of the fundamental and sum-frequency waves generated from the surface of a thin metal film in the Kretschmann configuration with coherent ultrashort pulses. We show that the spectra of reflected sum-frequency waves exhibit pronounced shifts for the incident fundamental waves close to the plasmon coupling angle. We also demonstrate that the scale of discovered plasmon-enhanced spectral changes is strongly influenced by the magnitude of the incidence angle and the bandwidth of the source spectrum.