Using scanning near-field optical microscopy for probing surface plasmon fields, we show that energy can be transferred from the plasmon propagating on one interface to the mode bound to the other interface of a thin metal film. This coupling is mediated by a narrow groove interrupting the silver film. Excitation of the second plasmon mode is detected by mapping the spatial intensity modulation induced by the interfering fields of the two plasmons. A quantitative analysis provides detailed knowledge about the degree of coupling. Our interpretation of the experimental results is confirmed by a numerical simulation of the field distribution.