Photonic circuits can be much faster than their electronic counterparts, but they are difficult to miniaturize below the optical wavelength scale. Nanoscale photonic circuits based on surface plasmon polaritons (SPPs) are a promising solution to this problem because they can localize light below the diffraction limit(1-8). However, there is a general trade-off between the localization of an SPP and the efficiency with which it can be detected with conventional far-field optics. Here, we describe a new all-electrical SPP detection technique based on the near-field coupling between guided plasmons and a nanowire field-effect transistor. We use the technique to electrically detect the plasmon emission from an individual colloidal quantum dot coupled to an SPP waveguide. Our detectors are both nanoscale and highly efficient (similar to 0.1 electrons per plasmon), and a plasmonic gating effect can be used to amplify the signal even higher ( up to 50 electrons per plasmon). These results may enable new on-chip optical sensing applications and are a key step towards 'dark' optoplasmonic nanocircuits in which SPPs can be generated, manipulated and detected without involving far-field radiation.