As the newly-regenerated Doctor arrives on Earth, so does the Nestene too.

UNIT's attempts to locate the meteorites are hampered by the mysterious plastic Autons, being constructed by Channing at a nearby factory, while the Doctor makes another attempt to escape from the hospital.

Ransome manages to escape from the plastics factory and alert UNIT, prompting the Doctor and the Brigadier to start an investigation.

With UNIT's investigation being blocked by the Replica of Scobie, the Doctor and Liz visit Madame Tussaud's to try and find out more as Channing prepares to activate the Autons.

The Doctor and Liz are summoned to the Wenley Moor Research Centre by the Brigadier, after several staff members suffer nervous breakdowns.

The Doctor attempts to convince people there are monsters in the caves, prompting Major Baker to start shooting at a reptile man.

The Silurians contact Quinn for help in retrieving the creature while the Doctor and the Brigadier organise a search of the moors.

With Quinn dead, the Brigadier decides to take an armed party into the caves, prompting the Doctor to contact the Silurians himself.

Worried about the Old Silurian's growing friendship with the Doctor, the Young Silurian has the scientist infect Baker with a virus and then release him.

While the Doctor and Liz try to find a cure to the virus spread by Baker, the Young Silurian kills the Old Silurian and takes control of his people.

While Liz distributes the cure to the virus, the Doctor discovers the Silurians now plan to wipe out humanity by destroying the Van Allen Belt.

While the Brigadier investigates a loss of contact with Mars Probe 7, the Doctor realises signals coming from the probe are of extra-terrestrial origin.

UNIT retrieve the returned Recovery 7 but then Carrington hijacks it while the Doctor and Liz try to decode the signal he sent.

Carrington is holding the astronauts at a secret location but when he takes the Doctor, Liz and the Brigadier there they find they have been kidnapped.

Liz has been captured by Reegan's men and the Doctor begins to suspect Taltalian is more involved than he appeared.

The Doctor decides to take Recovery 7 back into space to search for the missing astronauts but Reegan sabotages the launch.

The Doctor speaks with the alien captain and learns the astronauts sent to Earth were ambassadors. However, before he can alert the authorities he is kidnapped by Reegan.

Carrington is planning to expose the aliens on live television and call for an interplanetary war and the Brigadier must rescue the Doctor and Liz in time to stop him.

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!

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?