The propagation and filtering of surface plasmon polaritons in metal-insulator-metal nanosandwiches are investigated by using finite-difference time domain simulation. We study the optical transmission of a nanoscale waveguide coupled to a cavity situated either in the vicinity or in the interior of the waveguide. Depending on whether the cavity is inside or at the side of the waveguide, the transmission spectrum displays respectively peaks or dips which occur at the same frequencies. We study the dip and peak frequencies in the transmission spectrum as a function of the geometrical parameters of the cavity and the thickness of the metallic gap separating the guide from the cavity.