Conductive-atomic force microscopy has been used to perform nanoscale current imaging of Ni-ion-implanted polythylene terephthlate films. A reduction in bulk sheet resistivity, as the Ni dose is increased, is found to be accompanied by an evolution in local conductivity from a spatially homogeneous insulator to an interconnected network of conducting Ni crystallites. The crystallites have a mean dimension of 12.3nm, confirmed by x-ray-diffraction analysis.