Sheila Dunn

UNIT are at a project run by Professor Stahlman who intends to penetrate Earth's crust. But the drilling has already released something deadly from beneath the Earth's surface.

The green slime oozing from the drill head is transforming men into monsters. Stahlman refuses to listen to the warnings and resorts to sabotage so that his project may continue.

In the parallel world, the drilling project is about to penetrate the Earth's crust. The Doctor, arrested as a spy, is unable to convince his captors of the danger that faces them.

In the parallel world, Earth’s crust has been penetrated, releasing terrible forces from within the Earth. The Doctor must escape back to his world to prevent the same catastrophe.

Transported to a parallel world, the Doctor discovers that England is under military rule and the drilling project is at a more advanced stage.