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

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 their fellow 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 for 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 retrieves 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.

The Doctor and UNIT investigate strange happenings at a top secret government drilling project headed by the pompous Professor Stahlman.

The Doctor begins to suspect that penetrating the earth's crust will create more Primords but Stahlman sabotages the computer to stop him proving it.

The Doctor finds himself in a parallel universe where Britain is a dictatorship and the Inferno Project is a slave labour camp policed by his friends' counterparts.

The Doctor attempts to convince the alternate Inferno staff that penetrating the earth's crust will cause a disaster but instead is imprisoned with an infected Bromley.

The earth's crust has been penetrated and, while the Doctor and Greg try to contain the disaster, the Doctor begins to realise the planet has passed the point of no return.

With the parallel earth doomed, the Doctor asks his friends' counterparts to help him return to his own world but the Brigade Leader only sees the TARDIS as a means of escape.

The Doctor is back in his own universe and must stop Stahlman before his earth suffers the same fate as the parallel one.