From a microscopic point of view, we theoretically investigate fishnet metamaterials. We formulate the construction of
the fundamental Bloch mode by tracking the flows of energy through the fishnet structure. The analysis is supported by a
closed-form semi-analytical model based on surface-plasmon coupled-mode equations. The model provides an accurate
formula for the fishnet refractive index, including the real (negative-valued) and imaginary parts. The model simply
explains how the surface plasmon modes couple in the structure and it shines new light on the fishnet negative-index
paradigm at optical frequencies. It possesses broad flexibility in geometrical and material parameter tailoring of fishnet
properties, even including the gain-assisted case.