This story introduces the United Galactic Federation of Planets, which would form the setting for a small number of wildly inconsistent stories about future history, until the novels came along and tried to retcon it all into some kind of sense and set a bunch of more consistent but less interesting stories there.
The Doctor, to save Jo from the disappointment of going on a date with the obviously-closeted Mike Yates, takes her on a test drive of his newly-restored wicked-awesome vintage TARDIS. As usual, he drops it off a cliff while trying to park. So now they have to get involved in whatever the locals are doing.
The locals are a planet-wide medieval kingdom of 12 people, who just learned about the rest of the galaxy a generation ago when an Earth girl crashed there and got knocked up by the old King. The new King, named Peladon, the same as his planet, wants to join the Federation, because he's half-human on his mother's side, which he mentions a lot (and it's true; his father was actually the Second Doctor). The half of his council that sits on the left (which is only one person, the Chancellor) likes the idea, but the other half (the High Priest) is against the idea. Then the Chancellor gets eaten by a monster.
Meanwhile, the Federation arrives to consider the application. There are four delegates, from the farthest reaches of the galaxy—Arcturus from Arcturus, Alpha Centauri from Alpha Centauri, and natives of two planets around the closest inhabited star to Alpha Centauri, an Ice Lord and a human. The Earth guy doesn't show, so the Doctor is able to pretend to be Earth from Earth. But they won't allow Jo in because she's a girl, so he tells them she's actually Princess Federation from the Federation, because republican federations have princesses, right? Alpha Centauri is friendly to admission, Arcturus is against it, and the Ice Warrior, who's surprisingly not named Mars, is the swing vote.
It's very transparently Brentry. King Peladon is the People of Britain, who want to join Europe , and his Labour Chancellor has arranged for the negotiations, but the evil Tory High Priest wants to sabotage things because some Tories just can't let go of the old ways. Arcturus is France, openly hostile, while being secretly even more hostile, because if Peladon doesn't join, his people can enslave them and steal their mineral wealth without the interference of EEC regulations. The Ice Warriors are the West Germans, apparently unfriendly, but ultimately the strongest force for reason and good, and our best potential allies if we can get over what their ancestors did. And Alpha Centauri is… well, Alpha Centauri is a 6-foot walking penis, and I really have no idea how that fits the EEC allegory… maybe Belgium?
Anyway, France and the Tories work together on a long series of nonsensical plans from forging evidence to assassinating everyone to tricking the Doctor into a deadly trial by combat.
When the last one fails, Arcturus decides to just openly shoot the Doctor, although it's not even clear why this would help him, and it just leads to him getting exposed and shot.
When that doesn't work, the priest, who controls the entire army (six guys with pikes), decides to just take the king hostage and force the delegates off the planet, which actually probably would have worked if his dumber previous plans didn't all come back to bite him in the arse, literally.
There's an ancient superstition about a monster named Aggedor who protects the royal family. It turns out that Aggedor is actually the name of a species, not a single monster, and they're real, and they're not quite extinct, and the priest has found and trained one so it can eat all the pro-Federation people to convince the King that the Federation is a bad idea. Later, after the priest tries to get the Doctor killed, the Doctor escapes and runs into the monster and hypnotizes it. And right when the priest has finally won with his coup d'etat, the Doctor comes back with the hypnotized monster, and it rears up at the priest, who falls down dead, and everyone lives happily ever after.
The Love StoryEdit
Peladon seems to be flirting with Jo, and she's definitely into it. But then it turns out he doesn't actually want her body, he just wants a human advisor to help him modernize his kingdom, like his dad had. Poor Jo, she never gets any action. At least Peladon isn't hitting on her because he's still in denial about being gay.
Eventually he asks her to marry him, and she says no, because she's not really a princess. But maybe he really did fall in love with her? It's hard to tell. Oh, the feels, the greatest romance in history, dashed on the rocks. Oh well, soon she'll get a hot ecoterrorist.
The Racism StoryEdit
The Doctor believes the Ice Warriors are behind the whole thing because they're Ice Warriors. They explain that they've actually been strict pacifists for almost two millennia, and he checks up on the history and discovers that's true, but he still doesn't trust them.
This is clearly supposed to be a story about not letting our perfectly justified anti-Nazi sentiments turn us against the even-more-anti-Nazi generation running West Germany today. But it really comes across as the Doctor being racist and thinking Ice Warriors are always chaotic evil.
After enough of his shit, the Ice Warriors decide they can't trust the Doctor either. Which they really shouldn't, because he's transparently lying about who he is, and about everything else, not to mention openly mocking both Peladon's religion and the Federation's process. So, while they should be working together to solve the problems, they're busy capturing and interrogating and escaping each other.
Fortunately, the Ice Warriors shoot Arcturus before Arcturus can shoot the Doctor, which convinces the Doctor that they really are pacifists now, and they get along after that.
Come to think of it, the two male Ice Warriors clearly share a single bed, so maybe it's homophobia the Doctor's overcoming here, not racism?
With the Priest eaten by the friendly monster and Arcturus shot by the friendly Martian pacifist, there's nobody to stand in the way of Peladon having his official coronation and then joining the Federation. The Doctor decides the Time Lords must have lied about fixing his TARDIS and sent him here to arrange everything, but he's so happy with the results that he wants to stick around for once anyway to see everyone live happily ever after, because he hasn't seen a coronation since Clara was crowned as Queen Victoria.
But as it turns out, the Earth delegate wasn't missing, just late. And it's not a man named Earth of Earth, but a woman named Amazonia of the Amazon jungle on Earth. So the Doctor has to run away after all.
Traditionally, about 3/4ths of fandom considers this the highlight of Pertwee Who, while the other 1/4th hates it and points out how ridiculous Alpha Centauri looks. The truth is, it is deeply cheesy, but it's good cheese , if you can just relax and enjoy it.
Yes, the main plotline, most of the twists, the entire setting, and the romantic subplot are all laughable, but they're built around a cast of very diverse characters whose motivations are always relatable, including aliens who are actually alien—and, best of all, the Ice Warriors, who are very different but still recognizably the descendants of the ones who regularly failed to invade Earth 2000 years earlier.
Also, the big scary monster is so cute. How could you not love him?
- Episode 3 had Doctor Who's worst ratings ever, because a coal miner's strike meant large parts of the country had no power for their telly. They actually edited together a special recap of episode 3 to show before episode 4 the next week. Can you imagine them doing that in the 80s, or the 2010s?
- The TARDIS Data Core insists that it's a myth that Izlyr is an Ice Lord, because nobody says "Ice Lord" during the story. Sure, he's called "Lord Izlyr", and he's in charge, and he wears different armour from the regular Ice Warrior guy, which are all three of the criteria that distinguish Ice Lords and Ice Warriors in The Seeds of Death, and Barry Letts called him an Ice Lord in an interview, but we don't actually have enough evidence to believe that he's an Ice Lord.
- After appearing in this story and its sequel to convince everyone that Germans are good and England should join the EU, the Ice Warriors disappeared from the show for decades. Next time they appeared, they were evil again, and England left the EU. Coincidence? Yes, even if Boris Johnson does look kind of like a 6-foot-tall walking penis, it's almost certainly a coincidence; after all, they appeared a zillion times in the comics and novels in the late 90s as even more good, and yet the UK-DE stock market merger failed.