In this story, the Doctor and Jo mistakenly land on the ledge of a windy mountainside, and enter the citadel of Peladon. There, they are mistaken (and quickly assume the role of) the Earth delegate. Peladon is a planet under a feudal monarchy applying for admission to the Galactic Federation, and the delegates from Alpha Centauri, Arcturus, and Mars are assessing whether or not to accept Peladon's entry. The Doctor is the chairman delegate while Jo becomes Princess Josephine of TARDIS, a royal observer.
Even though Peladon's Grand Council decided for Federation membership, there's a clash of ideals. Chancellor Torbis is for progress, to bring Peladon out of primitive superstition, while the high priest Hepesh is worried that the ancient traditional ways will be swept away, with Peladon being a slave to the Federation. Torn between the two is the young and weak-minded king, also named Peladon, who was raised by both men. He is all for joining the Federation, but Torbis's mysterious murder is the first of many crises to arise. Hepesh believes it's a sign that the spirit of Aggedor, the royal beast of Peladon, is angry. With Torbis dead, Hepesh solely has the king's ear. The Doctor puts the problem before the king neatly: "Your majesty is an enlightened ruler, but who will your people believe, King Peladon or Aggedor?"
Many crises occur in this episode, including two attempted murders. And Hepesh is dead set on keeping Peladon the way it is, and to that end, he sees the Doctor, who is the chairman delegate, as his foremost enemy, and conspires to have him killed, or fearing reprisal from the Federation, expelled. Yet the Doctor decides to stay on and help the besieged king while solving the mystery of whether Aggedor is a ghost or a manifestation that's "solid hairy fat."
Of the delegates, the Doctor mistrusts the Ice Lord, Lord Izlyr. After all, the Ice Warriors, who in both previous encounters, tried to conquer Earth, so it's no surprise that he suspects them when someone attacks the delegate from Arcturus, plus Jo finds evidence pointing to that. Yet the Ice Warriors mistrust Jo because of the attraction Peladon has for her, and because Peladon's mother was from Earth, think that a possible marriage has political ramifications, giving Earth an edge in Federation affairs. And the Federation cannot override local laws, so there's a legal roadblock as well.
Peladon (the king) becomes smitten with Jo and wants to marry her, but Jo, though fond of him, stands firm as a neutral observer. Later, when the Doctor's life is in danger, she abandons all restraint and pleads with the king to save him, yet he is torn again by being loyal to the laws of the planet and his feelings for Jo.
This story was influenced by Britain's entry into the Common Market, and it's not at all bad. The delegates are creatively realized, but the high-strung but pacifistic Alpha Centauri is my favourite. Due to the body shape of this green hermaphrodite hexapod, with a round face that's one giant eye, it was referred to director Lennie Mayne as a "giant green d--k." A yellow jacket covered it for the story, but it was referred to as the "d--k in the jacket." Its high-pitched feminine voice which becomes hysterical also gives it personality. Other influences include Star Trek's "Journey To Babel" episode, and as for Aggedor, the Hound of the Baskervilles is an apt comparison.
And the lullaby the Doctor uses was originally spoken in the story The Daemons. Here, the Doctor croons it, using the Christmas carol "God Rest Ye Merry Gentlemen" as a melody.
David Troughton (Peladon) is the son of the second Doctor, Patrick Troughton, and later appeared as Egan in the Stranger series.
This story is either regarded as one of the best or worst of the Jon Pertwee era. I see it as inbetween, definitely not the worst, but good, not great. Its sequel, The Monster Of Peladon, is actually better.