The Guardian

The Time Lords discover that a secret file on the Doomsday Weapon has been stolen. They are convinced that The Master has taken it and because it is against their rules to get involved with the affairs of others, they decide to use The Doctor to investigate and to retrieve it for them.

Meanwhile, on Earth, The Doctor, who is still stranded on Earth, has managed to build himself a completely new dematerialisation circuit for his TARDIS. After fitting it to his time machine he leaves Earth, with Jo Grant, for a short trip. He is though completely unaware that the Time Lords have in fact taken over the TARDIS and that they are controlling his destination.

They arrive on the planet Uxarieus in the year 2472. Shortly, after arriving, on what The Doctor believes is a desolate planet, they are spotted by a colonist, and taken to the colonists' main dome where they meet their leader, Robert Ashe and his daughter Mary. The Doctor and Jo discover that the colonists are struggling to survive and the colony is in danger of starving to death as all their crops wither and die for no apparent reason. They also learn that as well as the beleaguered colonists there is also living on the planet an indigenous telepathic race that the colonists call Primitives. On top of this two of the colonists claim to have seen giant lizards in the night.

The Time Lords

That night, two colonists, Leeson and his wife, are attacked and killed by what seems to be one of the giant lizards. Another colonist, who witnessed the attack, informs the colonists' leader that their gunfire had no apparent effect and that the lizard then vanished. The Doctor is suspicious and decides to investigate Leeson's dome himself. There he discovers that, unknown to the colonists, members of the Interplanetary Mining Corporation (IMC) have also arrived on Uxarieus and are doing a mineral survey of the planet. He is taken to the IMC spacecraft where he meets Captain Dent. The Doctor learns that IMC are determined to exploit the planet as they have discovered large deposits of the mineral duralinium which will increase IMC's profits. The Doctor also deduces correctly that IMC have been organising the attacks on the colonists. Captain Dent's second in command, Morgan, has been using a Mark 3 servo robot, fitted with large lizard-like claws, to frighten the colonists so that they will then decide to leave Uxarieus. Realising that The Doctor is a threat to IMC's plans Morgan accompanies The Doctor back to Leeson's dome where he attacks him with one of the robots. The Doctor though manages to escape and returns to the main dome to warn Robert Ashe about IMC's plans for the planet.

At the colonists' camp it is agreed that there is a need to send for an Adjudicator from Earth to decide who has the right to stay on the planet. When trying to find proof that IMC are behind the monster attacks Jo, and one of the colonists called Winton, are captured by IMC. Winton manages to escape and The Doctor manages to persuade Captain Dent to release Jo. However, the security guard, escorting her back to The Doctor, is killed by a Primitive and Jo finds herself being taken into an underground city.

The Doctor and Jo in the TARDIS

On learning that Jo has been taken by the Primitives he goes to the underground city himself he is reunited with Jo and placed in a cell-like room containing a mural painted around the room which shows a once-powerful race regressing to performing sacrifices to a machine. The Doctor and Jo are eventually taken by alien priest-like beings to a room containing a reactor. A malformed dwarfish figure, called The Guardian, then emerges from the wall. The Guardian recognises that The Doctor has great intelligence and so lets them both go free.

Back at the colonists' camp they discover that the Adjudicator has arrived. The Doctor though is horrified to find The Master posing the Adjudicator. After arranging a meeting between the colonists and IMC, to decide on their respective futures, The Master rules in favour of IMC. The Master, though, seems far more interested in the Primitives' city and gets Robert Ashe to tell him more about it, hinting that he could change his mind if he learns more about the dwellings.

Curious as to why The Master is on Uxarieus The Doctor and Jo investigate his TARDIS - which is disguised as the Adjudicator's spaceship. Inside they find mineral reports relating to many planets and the ID card for the real Adjudicator. Unfortunately, while searching Jo breaks a security light-beam warning The Master that there are intruders in his TARDIS and so he traps them inside. The Master then forces The Doctor to take him to the Primitives' underground city. Jo is left prisoner in his TARDIS, under threat of death, to be his guarantee of The Doctor's co-operation. However, as they exit the Master’s TARDIS The Doctor drops the key, he used to enter The Master’s TARDIS, onto the ground. This is eventually found by two members of IMC who then free Jo.

Meeting the Colonists

While inside the underground city The Doctor learns from The Master that the primitives were once an advanced civilisation. However, before falling apart they built a super-weapon but it was never used - and he wants to claim this weapon for himself as he intends to use the threat of the weapon to hold the galaxy to ransom. The Doctor though rejects The Master's plans stating that absolute power is evil and corrupting. When it is realised that the Guardian's machine is in fact the Doomsday Weapon The Doctor also realises that the radiation emissions from the machine is what has caused the decline of the Guardian's race and also the crop failures that the colonists have been experiencing.

The Guardian then appears and The Master reveals his plans stating that he wants to restore their civilisation to its former glory and then rule like a benign god. The Doctor argues against him by pointing out that the weapon has never brought any good. This persuades the Guardian to allow The Doctor to destroy the weapon rather than let it fall into The Master's, or anyone else’s, hands. After activating the self-destruct the two Time Lords get clear just in time as the machine explodes blowing up the Primitive’s city.

On their way back to the colonists’ camp The Doctor is reunited with Jo and they discover that IMC have taken over and have forced the colonists to leave. However, because their spaceship is no longer capable of space flight Robert Ashe sacrifices himself by piloting their spaceship alone – leaving all the colonists behind. As he expected as it takes off the spaceship explodes. This prompts IMC to assume that all the colonists were aboard their spaceship and so have been killed. Therefore IMC are unprepared when they are then attacked by the colonists and so are forced to surrender.

With the news that another Adjudicator has been sent for, and with The Master having escaped in his TARDIS, The Doctor and Jo return to Earth where, as far as The Brigadier is concerned, no time has passed since their departure.