  Within the framework of the scalar-tensor theory we consider a hill-climbing
inflation, in which the effective Planck mass increases in time. We obtain the
Einstein frame potential with infinitely long and flat plateau as we approach
towards the strong coupling regime, together with a run-away vacuum in the GR
limit of the theory. The inflation ends with the scalar field rolling down
towards infinity, which at the effective level indicates the massless scalar
field domination in the Universe. In this scheme we assume that the inflaton is
a dark particle, which has no couplings to the Standard Model degrees of
freedom (other than the gravitational ones). We discuss the gravitational
reheating of the Universe together with its implications on the predictions of
the model, including possible amplification of primordial gravitational waves.
Our model for the first time realizes explicitly the enhancement of the
primordial gravitational waves in the dark inflation scenario.
