Half way in, all spoilers, folks.

Episode 4:

 The Doctor tells Ashe that he has to go rescue Jo; Ashe says in the past they have bought back captured colonists from the primitives with food. As The Doctor goes to leave, a spacecraft arrives and lands. Ashe says it must be the Adjudicator.

 On the IMC ship, Captain Dent tells Winton that taking his ship won’t reflect well on their case in the Adjudicator’s eyes. Winton finds the projector that creates the illusion of the giant reptile, as well as the giant claws for the robots.

 Ashe receives a call from the Adjudicator, who says that he must summon all parties for him to meet with.

 Jo is taken into the city and meets with another alien, looking even stranger than the others.

 Leeson’s brother wants to kill Morgan and Dent, but Winton stops him. Morgan says he’s got proof in a secret drawer, but pulls out a weapon and takes Winton hostage.

 The Doctor arrives at the secret door and is greeted by a group of primitives. They open the door for him and he enters.

 The Adjudicator arrives at the colony and is greeted by Ashe and his daughter; the Adjudicator is, as I suspected, The Master.

 The Doctor is brought to where Jo is and the door is shut behind them. Jo tells him about the other alien. Looking at the technology, The Doctor posits that the primitives were once an advanced race. They find a graphic chronicle depicting some sort of catastrophe.

 The pale skinned native (not alien, The Doctor, Jo and the colonists and IMC are the aliens) comes in. The being looks at a picture on the chronicle that seems to be a sacrifice, then points at Jo and then the picture before departing.

 The Adjudicator listens to the colonists’ accusations. They cannot provide any evidence as the IMC crew has destroyed the projector and claws. The Adjudicator then asks to hear from Captain Dent.

 The Doctor distracts a primitive with sleight of hand and Jo grabs his spear while The Doctor disables him. They slip out, avoiding several other pale skinned natives, accompanied by the taller, previously seen green skinned primitives. The primitives quickly give chase but The Doctor and Jo are shortly surrounded and escorted to a room that has a strange machine.

 Inside the machine is a diminutive pale skinned native, who can speak. It tells them the city is forbidden and demands to know why they are there. Jo says she was brought there and The Doctor came to bring her back. The Doctor argues that she was brought as a captive and they just want to be allowed to leave.

 The primitive leader says they may leave, but if they ever return, they will be destroyed. (So, you know, they’re going to have to before the serial ends.) After they’re free of the city, The Doctor wonders if there are three races or just three mutations of the same native race.

 The Adjudicator says that he’s heard both sides and has been in touch with Earth; it seems there’s a faulty computer on Earth that assigned both “colonisation” and “mining” to this planet, so it is up to him to judge the case on the merits. As he is talking, The Doctor and Jo walk back in, Jo’s jaw dropping in surprise. The Doctor is more composed, and The Master ignores Jo’s outburst, “It’s The Master!”

 The Doctor and The Master and Jo step aside, the two Time Lords talking. The Master says that he has credentials, and The Doctor does not – in this day and age, they are essential and since The Doctor doesn’t have them, he could have him sent to Earth as a prisoner, and advises him to remain silent, if he doesn’t want that to happen.

 The Adjudicator returns to say that he sympathises with the colony, but since they’re unsuccessful, he rules in the favour of the IMC.

 Captain Dent and his crew celebrate with drinks on their ship.

 Winton advocates forcing the IMC men off the planet, and to break from Earth and declare as an independent republic. Ashe argues against it, but there’s no use.

 The Doctor tells Jo that The Master is on the planet for a reason, he just needs to find out why.

 The Adjudicator contacts Captain Dent, asking Dent and his officers to attend him in the main dome. It turns out that it’s Winton masking his voice, setting them up for ambush.

 The Adjudicator tells Ashe that there is likely no chance of an appeal to Earth being successful, unless there are any special circumstance, historical value, etc. Ashe tells him of the ruined city not far, how it was once of a great civilisation. The Master is very keen to hear about this.

 Norton is attempting to radio IMC to warn them of the ambush. One of the colonists finds him and they struggle. Norton kills him but the radio is destroyed. Norton arrives at the ambush and Winton tells him to join the ranks. Norton calls out to Dent as the IMC men walk in, and Winton shoots the spy.

 A firefight breaks out between colonists and IMC. The Doctor wants to go stop the fight, but The Master stops him, pointing a gun at them, saying the two of them are about to be victims of stray bullets… and the credits roll.

Episode 5:

 As The Master threatens The Doctor with the gun, Ashe and others run up and he quickly lowers the weapon. Ashe says they must do something and The Doctor asks, “Yes, we must, don’t you agree, Adjudicator?”

 Winton takes Dent captive, who orders his men to drop their guns. The colonists take the IMC officers away. Winton says they will declare their independence from Earth and The Master tells him if that is their intent, they will need someone to adjudicate on their behalf.

 He says that Ashe has told him that there are claims of historical importance in regards to the planet. If he can investigate these claims, perhaps he could reverse his ruling and find in favour of the colony.

 The Doctor tries to tell Ashe that The Master is an impostor, but the colony governor won’t listen. When The Doctor demands they check the Adjudicator’s credentials, The Master asks to see The Doctor’s.

 Jo and The Master talk about getting into The Master’s TARDIS (which is disguised as his spacecraft) to find some evidence to prove that he’s a fake. When Jo asks how they’ll get in, The Doctor produces a key and says that he swiped it when The Master first arrived on Earth.

 Dent and his crew are brought back to the ship and told to leave immediately. Dent gives orders to have Earth contacted to check out the Adjudicator, but the colonists contact the ship to let them know some of their explosive has been set up beneath the ship; if they don’t take off immediately, they’ll blow up the ship.

 The Doctor and Jo enter The Master’s TARDIS. A beeping sounds and The Doctor pulls out his sonic screwdriver, using it to detect an alarm beam. They get on the ground and crawl beneath it (Jo, for some reason, on her back, which seems a stupid way to crawl under a beam.)

 The IMC ship goes into ‘parking orbit’ when a message from Earth Control comes in. Dent gets confirmation that The Master is not the Adjudicator and orders the ship to ready to land on the planet.

 In The Master’s TARDIS, The Doctor finds a bunch of mineralogical surveys for different planets. They find the real Adjudicator’s credentials, but The Doctor pokes around, hoping to find out what The Master is up to. Jo stupidly walks out the door, tripping the alarm, which alerts The Master. He views them from a handheld device, and triggers a gas vent in the TARDIS. The doors close as they begin to choke and cough and collapse.

 Ashe consults with The Master about the primitive city. The Master learns that only The Doctor and Jo have been inside the city.

 The IMC ship lands about fifty kilometers from the main dome. Dent gives a rousing speech to his men about getting revenge for being embarrassed by a bunch of farmers.

 The Master revives The Doctor.

 “Well, Doctor, still pursuing burglary, eh? You know, when you stole my dematerialisation circuit, I decided to build in a few precautions.” – The Master, gloating, of course.

 He tells The Doctor he wants him to take him to the primitive city (told you they’d be going back); Jo is to be held prisoner on the TARDIS as insurance.

 Winton and Ashe argue over the IMC weapons and the Adjudicator’s presence and whether he’ll be helping or not. After Winton chastises two guards for almost dozing off, IMC troops take out the sentries. They skulk into the colony and take Ashe prisoner. They arm themselves with the weapons the colonists had confiscated and, once more, a firefight between the two forces begins.

 The IMC troops end the fight by bringing Ashe out and demanding the colonists surrender. Dent says he’s going to put them on trial for treason.

 The next morning, The Master takes The Doctor out of a tube-like cell (Jo being held in an adjacent one) to go to the city. He tells him that he has a device, all he needs to do is press a button and Jo’s cell will be flooded with lethal gas. As they depart the TARDIS, The Doctor drops the TARDIS key outside.

 Dent, declaring himself “legally appointed governor” of the planet opens the trial. Ashe and Winton are present, manacled, and charged with “destroying property and equipment belonging to the IMC, assault and murder of IMC personnel, trespass on a planet lawfully allocated to IMC and armed rebellion against the lawful representatives of the Earth’s government.”

 Dent finds them guilty and sentences them to execution, but offers to suspend the sentence if the colonists leave the planet immediately. Ashe protests, saying that it was old when they bought it, it wouldn’t survive another trip and it would be a death sentence for the whole colony. Dent says the trial is over and they better prepare for take off immediately.

 The primitives drop a boulder down on The Doctor and The Master’s buggy, but they escape the trap. They continue on foot.

 Mary Ashe argues with Caldwell over the safety of their ship. He obviously is having second thoughts about their chances of survival and she appeals to him.

 Dent sends men to fetch the false Adjudicator, he wants to question him on why he’s there, what he’s up to.

 The Doctor and The Master reach the door in the rockface. The Doctor says he has no idea how to get in and The Master waves his control device for Jo’s poison under his nose.

 Caldwell and Morgan find the key outside The Master’s TARDIS and use it to gain entry. They enter and see Jo and try to get her out, but they set off the alarm. The Master sees them on his device, and reaches to press the button to discharge the gas, as The Doctor protests… and the credits roll.

 Great cliffhanger, that one!

Episode 6:

 The Doctor kicks the device out of The Master’s hand, as well as his laser weapon. Before anything more can happen, the primitives come out and surround the two Time Lords.

 Morgan and Caldwell get Jo out and start questioning her where the Adjudicator is. She tells them that he and The Doctor went to the primitive city.

 The Doctor and The Master are brought into the chamber where Jo was held before. The Doctor explains the primitives don’t speak but are telepathic. He directs The Master’s attention to the frieze depicting the history of the primitives.

 Dent tells Jo that she has to go on the ship with the colonists. Caldwell helps her slip away, after she gets him to understand that she needs to find The Doctor and perhaps he can fix everything.

 The Master confides that he already knew about the history of the planet, having stolen files from the High Council of the Time Lords. The Master mentions the superweapon and his desire to have it, but the primitives enter. The Master shows The Doctor he has a gas bomb (but only one rebreather) and gives his nemesis a head’s up before he uses it. The Doctor holds his breath and they escape from the room.

 Ashe says Dent is sending them to their deaths, but the captain says his man has checked the ship out and it’ll fly. He tells Ashe he’s not worried what happens when they get off-planet.

 The Master leads The Doctor through the warrens towards the superweapon.

 Dent orders a guard to observe the colonists’ ship; if they try to disembark, he is to call for backup. After the IMC men leave the colony, we see Winton hasn’t made it aboard, though Mary Ashe argued with Morgan that he was already on the ship and had been for a while, working on the engines.

 Winton attacks the guard on observation duty. They fight.

 Dent contacts Ashe aboard to find out why they haven’t launched yet, but is told just some minor repairs. He says good bye and terminates the call.

 Caldwell and Jo arrive where the buggy The Doctor and The Master were using had been attacked by primitives. They get out to look around, but hear the launch of the colonists’ spacecraft. They turn to watch and are horrified as it explodes in mid-air.

 The Master and The Doctor reach a chamber that The Master says is the heart of the weapon. It stretches for miles around them. He operates a command console. On a viewscreen, he brings up Earth’s Sun and says with this weapon he could cause it to go supernova prematurely. The Master says the Crab Nebula was the result of the super race testing their weapon.

 Caldwell and Jo arrive at the entrance to the city, but cannot get in. After Caldwell says they’ll have to find another way in, a primitive comes out, but Caldwell jumps him from behind and he and Jo slip in.

 The Master plans to use the superweapon to hold the universe at ransom, to put himself in power over everyone. He proposes The Doctor and he join forces and rule together, each one half of the universe.

 Dent discovers Morgan has left for the primitive city and sends Morgan to bring him back. They talk about the colonists’ ship exploding and Dent says it’s all covered in his report, their asses are covered.

 The Doctor and The Master argue over ruling the universe. The Master gets angry and points his weapon at The Doctor, when the diminutive primitive comes out of the wall. He demands to know why The Doctor has returned, and why The Master is there.

 The Master says he wants to help them create a master empire, but The Doctor argues that he’s lying. The Doctor asks the leader if the weapon brought them any good, and the little man says that once the weapon was built, their race began to spiral downward. The radiation from the power source poisoned the planet.

 The leader says that The Master will only bring unhappiness and destruction to the universe were he to have control of the weapon. The Master points his weapon at the little primitive, only to see his weapon disappear from his hand.

 At the leader’s behest, The Doctor activates the self-destruct for the weapon (why this wasn’t done before if the leader knew it was of no good, I don’t get…). The city shakes and primitives run about as the two Time Lords run into Caldwell and Jo and the four of them escape, moments before the city blows up.

 Morgan and a crew of IMC troops show up and take everyone captive. Morgan tells The Master that they know he’s an impostor. Morgan plans to kill The Doctor, The Master and Jo, sparing Caldwell only because they need him, but the colonists appear and begin a firefight (hey, there’s a pattern…) with the IMC troops. The troops surrender after a number of casualties.

 The Master slips off in the chaos, and The Doctor and Jo give chase in a buggy. The Master makes it back to his TARDIS and takes off; The Doctor and Jo watch as it disappears.

 Back at the colony, Winton tells them that Ashe was the only one on the rocket; he piloted it so IMC would think everyone was on board. The colonists found the TARDIS for him, much to his and Jo’s delight. Caldwell stays on board with the colonists, saying he has no future on Earth. Caldwell, Winton and Mary (who’s awfully calm having just lost her father) watch as the TARDIS dematerialises.

 Back in UNIT HQ, the Brig is demanding The Doctor come back at once – only mere moments have passed there since the beginning of the serial. The TARDIS appears as if on command and Jo and The Doctor disembark.

 “Well, that was a short trip. You’ll never get that thing working.” – The Brig

 The Doctor tells Jo they’ve only been gone for a few seconds in the Brig’s timeline, but not to bother trying to explain, he’d never understand… and the final credits roll.

 A fun serial, lots of chicanery on The Master’s behalf, good characters, good story. Sinister IMC, mysterious primitives. A sold serial, all around.