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 parallel Inferno has broken through the Earth's crust, unleashing terrible seismic forces and dooming the planet. The Doctor hopes to return and save his world. But the bestial primords are closing in...

The Doctor is trapped in a parallel world that is going up in flames. Can he escape in time to save Earth in his own universe?

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

The Doctor and Jo head to Stangmoor Prison to investigate the Keller Machine, which drains the evil impulses from prisoners' minds, while UNIT are handling security at the first world peace conference.

The Doctor is recalled by UNIT to help investigate Cheng-Tiek's murder but is unaware that Chin-Lee is under the control of the Master.

Realising Chin-Lee has been using the power of the Keller Machine to carry out assassinations, the Doctor heads back to Stangmoor, only to find the Master has teamed up with Mailer to stage another riot.

With the Doctor and Jo being held prisoner, the Master convinces Mailer to help him steal the Thunderbolt.

The Doctor is forced to help the Master control the mind parasite while Yates finds out where the Thunderbolt has been taken only to be captured by the Master's men.

The Brigadier has recaptured Stangmoor but the Master still has control of the Thunderbolt and the Doctor must find a way to stop him and destroy the Keller Machine.