A theoretical model is suggested which describes plastic flow through stress-driven migration of high-angle grain boundaries (GBs) in metal-graphene nanocomposites. In the framework of the suggested model, stress-driven GB migration gives rise to the formation of wedge disclinations at GB junctions and edges of graphene inclusions. Energy and stress characteristics of stress-driven GB migration are calculated in several metals (aluminium, nickel and Gum metal). It is found that graphene inclusions strengthen metal-graphene nanocomposites. This is well consistent with experimental data reported in literature. Also, it is revealed that graphene inclusions in metal-graphene nanocomposites either hamper or enhance unstable GB migration and thereby grain growth driven by stress, depending on inclusion length.