<p>This paper investigates the effects of the interaction between an under-expanded supersonic jet and a flat plate on screech generation. The plate is installed parallel to the jet axis and an experimental investigation is made for several configurations corresponding to different radial distances of the plate, <i>H</i>, from the nozzle axis. Acoustic measurements are carried out to evaluate the plate impact on jet aeroacoustics. The results show that, when the radial distance is less than 1.5 nozzle diameters, the plate substantially alters the staging dynamics between modes A1 and A2 and between modes B and C. Specifically, the plate induces an extinction of modes A2 and C together with an extension of modes A1 and B. The extent of flow conditions for which the extinction of modes A2 and C is observed is dependent on the jet-plate distance, the smaller the distance the wider the range of flow conditions. Furthermore, the plate induces a frequency splitting of mode B for the smallest jet-plate distance, that is, <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(H=0.7D\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>H</mi> <mo>=</mo> <mn>0.7</mn> <mi>D</mi> </mrow> </math></EquationSource> </InlineEquation>. A time-frequency analysis is performed to explore in detail the staging dynamics between A1 and A2 modes. The study shows that the jet experiences a complete suppression of mode A2 for <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(H=0.7D\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>H</mi> <mo>=</mo> <mn>0.7</mn> <mi>D</mi> </mrow> </math></EquationSource> </InlineEquation> and an intermittent switch from A1 to A2 modes as the jet-plate radial distance is increased. Finally, schlieren measurements are conducted to assess whether the observed modifications of the screech behaviour might be associated with changes of the shock-cell structure. The analysis reveals that for <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(H=0.7D\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>H</mi> <mo>=</mo> <mn>0.7</mn> <mi>D</mi> </mrow> </math></EquationSource> </InlineEquation> the surface induces an attenuation of the amplitude of the secondary wavenumber peak of the shock-cell structure, thus providing a possible explanation of the A2 mode suppression observed in the noise spectra.</p>