England, the near future. UNIT is providing security and scientific support for the Stahlman project, an attempt to tap the Earth's core for energy. But things are already going wrong…

Strange deaths, men changing into monsters, hot green slime pouring out of the waste pipes – but Professor Stahlman is determined to press ahead with the Inferno project no matter what the cost!

The Doctor has vanished from the Inferno project… or has he? He finds himself in the same place, but everyone has changed… and the drilling is closer to its deadly conclusion.

In a parallel England, the Doctor tries desperately to convince his captors of the immense peril they are in. But they are convinced he is a spy. And penetration zero is getting close…

The Doctor has escaped the catastrophic destruction of a parallel world. But can he stop Stahlman before he destroys this one?