UNIT and the Doctor are at an experimental drilling project that is supposed to penetrate the Earth's crust.  A green liquid starts to exude from the drills that de-evolves anyone that touches it.

The Doctor, while working on his TARDIS console, is transported to a parallel universe in which England is ruled over by a dictatorship.  The drilling project in this universe is much more advenced in the process.  The Doctor is unable to prevent drilling.  When the Earth's crust is penetrated, lava erupts and destroys the world.  He is able to return to his own universe where he is able to shut down the drilling just in time to avert disaster.