The interplay of structural and electronic properties on plasmon modes was investigated for a thin Ag film grown at room temperature on Cu(111). Surface plasmons are confined within Ag grains, as indicated by the analysis of their dispersion relationship, which is dispersionless up to a critical wave-vector. Surface plasmon confinement is removed upon annealing at 400 K. The thermal treatment induces a flattening of the Ag adlayer with a merging of Ag islands, and, moreover, a strong enhancement of surface conductivity.