For nearly two decades, researchers in the field of plasmonics -which studies the coupling of electromagnetic waves to the motion of free electrons near the surface of a metal -have sought to realize subwavelength optical devices for information technology , sensing , nonlinear optics , optical nanotweezers and biomedical applications . However, the electron motion generates heat through ohmic losses. Although this heat is desirable for some applications such as photo-thermal therapy, it is a disadvantage in plasmonic devices for sensing and information technology and has led to a widespread view that plasmonics is too lossy to be practical. Here we demonstrate that the ohmic losses can be bypassed by using 'resonant switching'. In the proposed approach, light is coupled to the lossy surface plasmon polaritons only in the device's off state (in resonance) in which attenuation is desired, to ensure large extinction ratios between the on and off states and allow subpicosecond switching. In the on state (out of resonance), destructive interference prevents the light from coupling to the lossy plasmonic section of a device. To validate the approach, we fabricated a plasmonic electro-optic ring modulator. The experiments confirm that low on-chip optical losses, operation at over 100 gigahertz, good energy efficiency, low thermal drift and a compact footprint can be combined in a single device. Our result illustrates that plasmonics has the potential to enable fast, compact on-chip sensing and communications technologies.