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