Recent multi-wavelength observations of gamma-ray burst (GRB) afterglows revealed unexpected features such as the plateau in X-ray light curve, followed by a peculiar chromatic break. The standard model of afterglow production by the forward shock wave is difficult to reconcile with these data. This calls for revisiting the basic picture of GRB explosion. Theoretically, a long-lived reverse shock expected in the burst ejecta needs to be consistently described together with the forward shock even for a very general class of explosions. The mechanical model that we developed to describe relativistic blast waves meets this requirement. We perform detailed numerical simulations and calculate the emissions from both the forward and reverse shocks. We present the resulting X-ray and optical light curves, and discuss the characteristic features of the reverse-shock emission. We then propose a new model in which the forward shock is invisible and the afterglow is emitted by a long-lived reverse shock in the burst ejecta.