Inferno is the name of a top-secret drilling project to penetrate the Earth's crust and release a major new energy source.

A crisis develops when a noxious liquid leaks out as drilling progresses — the green poison has a grotesquely debilitating effect on human beings.

As the Earth's plight worsens, the Doctor is trapped in a parallel world, unable to rescue the planet and its inhabitants from the destructive force of Inferno...