A report on the doomsday weapon has gone missing from the Time Lords' files and they are convinced that the Master has taken it. The Time Lords (Peter Forbes-Robertson, John Baker, Graham Leaman) decided to use the Doctor to investigate. The Doctor has built himself a completely new dematerialisation circuit for the TARDIS. He fits it and the TARDIS, together with himself and Jo, promptly leaves Earth for the planet Uxarieus in the year 2472. When they arrive, they are captured by Leeson (David Webb), a colonist, and taken to the colonists' main dome where they meet their leader, Robert Ashe (John Ringham), and his daughter Mary (Helen Worth). The Doctor realises from the crop records that the colony is in danger of starving to death as all their crops wither and die for no apparent reason. On top of this, two of the colonists, Martin (John Line) and his wife (Mitzi Webster), have seen giant lizards in the night. The TARDIS is taken by the alien natives, the Primitives (Stewart Anderson, Derek Chafer, Les Clark, Pat Gorman, Emmett Hennessy, John Mc Grath, Walter Turner, Mike Stephens, Terry Walsh, Dinny Powell, Alf Joint, Mike Horsburgh, Les Clark, Valentino Musetti). 

That night, Leeson and his wife Jane (Sheila Grant), also see a lizard, are attacked and killed. David Winton (Nicholas Pennell) says that the lizard vanished, their gunfire having no apparent effect. A stranger called Norton (Roy Skelton) arrives at the camp. He claims to be the only survivor from another colony which was also attacked by the lizards. The Doctor is suspicious and investigates Leeson's dome. There he confronts a Mark 3 servo robot (John Scott Martin) belonging to the Interplanetary Mining Corporation - IMC. Its operator, Caldwell (Bernard Kay), is doing a mineral survey of the planet. He takes the Doctor to the IMC spacecraft where he meets Captain Dent (Morris Perry). Dent is sympathetic, but is privately determined to exploit the planet as it has heavy deposits of the mineral duralinium which will ensure that IMC's profits are high. Dent's second in command, Morgan (Tony Caunter), has been organising the attacks on the colonists, and Caldwell is upset that people have been killed. Morgan accompanies the Doctor back to Leeson's dome and attacks him with a robot fitted with large lizard-like claws. The Doctor manages to escape and returns to the main dome to warn Ashe. 

Meanwhile Norton, who is in league with IMC, kills Jim Holden (John Herrington), the colony's electrician, and sabotages the power supply, blaming it on one of the telepathic Primitives. Dent meets with Ashe and agrees to send for an Adjudicator from Earth to decide who has 'ownership' of the planet - the colonists or IMC. Jo and Winton decide to try and find proof that IMC are behind the monster attacks but are captured and chained to an explosive charge in one of the Primitives' dwellings. 

Winton is shot by IMC security guards as he escapes but manages to get to Caldwell's hut where the mineralogist helps him. Winton then decides to attack the IMC ship. The Doctor goes to speak with Caldwell who forces Dent to have Jo returned to the ship. Security Guard Allen (Stanley McGeagh) is, however, killed by a Primitive and Jo is taken into their underground city. 

The Adjudicator arrives and turns out to be the Master (Roger Delgado). He arranges a meeting between the colonists and IMC to decide on their respective futures. The Doctor goes to the city and is taken below by the Primitives. He is placed in a cell-like room with Jo. There is 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 (Roy Heymann, Stanley Mason, Antonia Moss) to another room which contains a reactor. A malformed dwarfish figure emerges from the wall. This is the Guardian (Norman Atkyns) who recognises that the Doctor has great intelligence and lets him and Jo go free. 

Back at the main dome, the Doctor is horrified to find the Master posing as the Adjudicator. The Master rules in favour of IMC. He seems interested in the Primitives' city and gets Ashe to tell him more about it, hinting that he could change his mind if he learns more about the dwellings. Winton leads the colonists against IMC and the colonists take control. 

The Doctor and Jo investigate the Master's TARDIS - which is disguised as the Adjudicator's ship - using the key that the Doctor obtained from Professor Philips [Terror of the Autons]. They find mineral reports relating to many planets and the ID cards for the real Adjudicator, Martin Jurgens. Jo breaks a security light-beam and the Master floods his ship with gas which renders the Doctor and Jo unconscious. 

Dent discovers that the Master is an impostor. IMC guards (Bob Blaine, Les Clark, Brian Gilmar, Brian Justice, Keith Simon, Stewart Stephens, Mike Stevens, Barry Stephens, Jay Neill, Dinny Powell, Mike Horsburgh, Alf Joint) attack the main dome and take control. Dent insists that the colonists leave the planet in their own spacecraft which, according to Ashe, is not spaceworthy. 

The Master and the Doctor go to the Primitive city - Jo is left prisoner in his TARDIS under threat of death to be his guarantee of the Doctor's co-operation. As they leave the ship, the Doctor drops the TARDIS' key on the ground. This is found by Caldwell and Morgan who free Jo. 

After being taken prisoner and escaping from the mural room, the Master and the Doctor make their way to the reactor room. The Master explains to the Doctor that the planet was once home to a super-race who developed a doomsday weapon that was never used in anger. The crab nebula was the result of the weapon being tested. The Master wants to use the threat of the weapon to hold the galaxy to ransom. 

The colonists (Bob Blaine, Monique Briant, John Caesar, Leslie Conrad, Ian Elliott, Charles Finch, Brian Gilmar, Ken Halliwell, Clay Hunter, Jay Mc Grath, Alan Peters, Charles Pickess, Terry Walsh, Max Diamond, Valentino Musetti, Bill Horrigan) board their ship, except Winton, who attacks the guard, Rogers, (Terry Walsh) posted to ensure that the colonists stay there. Caldwell and Jo head for the Primitives' city. The colonist's ship takes off and explodes. Caldwell is sickened by this as he checked the engines for them and claimed that they were okay. He and Jo get into the city. 

The Doctor and the Master confront the Guardian. The Master states that he wants to rule like a benign god, but the Doctor points out that the weapon has never brought any good. Even the soil of the planet has been poisoned by radiation from its power source. The Guardian pronounces that the Master is not fit to be a god and guides the Doctor to operate the weapon's self destruct mechanism. The Doctor and the Master hurry away, meeting Jo and Caldwell on the way. They escape before the Primitives' city blows up. Outside they are captured by IMC, but IMC are ambushed by the colonists, who had left their ship before it took off. Ashe sacrificed himself by piloting the ship and putting IMC off their guard. 

The Master leaves in his TARDIS, Caldwell decides to stay and help the colonists, another Adjudicator is sent for and Winton has had the Doctor's TARDIS - which was found in one of the Primitives' dwellings - brought to the main dome. The Doctor and Jo return to Earth where, as far as the Brigadier (Nicholas Courtney) is concerned, no time has passed since their departure.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}