A broadband plasmonic bandstop filter with a single directly connected rectangular ring resonator is proposed. Inside this resonator, which is constructed using two nonidentical branches, both ring resonance and Mach-Zehnder interference occurred. A broad stopband can be realized by appropriately adjusting the resonant wavelengths of these two interferences. An equivalent circuit model is established to analytically describe the performance of this structure and to systematically target its design parameters to achieve the minimal transmittance in the stopband. An example of the plasmonic bandstop filter with its dimensions of 200 nm x 522 nm and stopband edge wavelengths of 1300 and 1800 nm is numerically demonstrated.