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.

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.

When someone tries to assassinate diplomat Sir Reginald Styles, UNIT investigate the possibility his attacker came from the future.

Jo uses the guerrillas' time machine to travel to the 22nd century where the Controller tricks her into telling the Daleks where she's come from.

The Doctor forces the guerrillas to take him to the 22nd century where he is soon captured by the Controller.

The guerrillas infiltrate the Daleks' base to rescue the Doctor and Jo and ask them to return to their own time and alter history before the 20th century is plunged into a Third World War.

England, the 12th century. Robber baron Irongron is recruited by Linx, a Sontaran warrior whose starship has been forced down on Earth. Linx is forced to use a time-travel device to steal resources and scientists from the future to repair his ship, but in doing so involves UNIT and the Doctor...

The barren planet Exxilon, some time in the future. The TARDIS is forced down by a total power failure and the Doctor and Sarah meet a stranded Earth crew who came here to mine for priceless medical supplies. But what is causing the power failures? And why have the Daleks come here too?

The Doctor and Bellal continue to penetrate the City's defences while Sarah and Jill try to stop the Daleks leaving with the parrinium.