The prehistoric Earth is dying. Thunderclouds roll across the skies, cloaking the land in darkness. The seas crash and boil as the rain turns to acid. The remnants of the Silurian race place themselves in suspended animation, deep below the surface.

One day they will awaken and reclaim their world…

The TARDIS has landed on the Galapagos Islands, a desolate outcrop of rocks shrouded in mist and fear. In the settlement of Baquerizo Moreno, there are rumours that prisoners have been mysteriously disappearing from the gaolhouse. A fisherman has been driven insane by something he saw in the caves. And the Doctor and Evelyn are not the only new arrivals; there is also a young natural philosopher by the name of Charles Darwin…