The one with a talking dildo alien.
The Doctor thinks he's managed to fix the TARDIS at long last. Taking a test flight, he and Jo find themselves on the planet of Peladon, where a new king is about to be crowned and the Galactic Federation are assessing the planet for entry. Only the human delegate is missing - so of course the Doctor is immediately assumed to be him, a role he dives into with enthusiasm.
The other committee members - the giant, camp, eyeball-octopus-thing Alpha Centauri, Arcturus the Brain in a Jar, the Ice Warrior lord Izlyr and his subordinate, Ssorg - are worried about the death of Chancellor Torbis, the new king's advisor. High Priest Hepesh opposes entry into the Federation and attributes Torbis's death to the displeasure of Aggedor, the sacred monster of Peladon, who is most certainly not a much bigger ALF. The Doctor, meanwhile, concludes that a saboteur is at work.
Initial suspicion naturally falls on the Ice Warriors - after all, they've been villains twice already - but of course it's Hepesh himself. Arcturus, one of the delegates, has convinced Hepesh that the Federation will exploit Peladon for its mineral wealth, whereas in reality his own government plan to do so once Peladon has rejected entry. Ssorg, defending the Doctor, kills Arcturus, but Hepesh gets away, fleeing into a network of tunnels behind the giant statue of Aggedor.
Hepesh leads a coup and attempts to force the king to reject entry into the Federation once and for all. The Doctor arrives with the real Aggedor, a mountain beast thought extinct which Hepesh had trained to obey him, and which he had used to spread the story of the curse. Hepesh orders Aggedor to kill the Doctor. But the Doctor has already met and befriended Aggedor via hypnosis, and it turns on Hepesh instead.
Peladon tries to convince Jo to stay and marry him, having fallen in love with her over the course of the story. Jo reluctantly declines, though has enough second thoughts that the Doctor must talk her into returning to Earth. They head off to watch the coronation before their departure, only to run across the other delegates meeting with the actual Earth delegate, who has arrived late and is indignant that someone has been impersonating her. The Doctor and Jo beat a hasty retreat to the TARDIS, where they VWORP away just as the delegates enter the room looking for them.
- Alas, Poor Villain: Hepesh's death seems to devastate everyone present, especially Peladon.
- Arranged Marriage: The other alien delegates believe that this is why "Princess Jo" has accompanied the Doctor. Jo later sets them straight.
- Author Tract: The story was allegedly pro-European Union.
- Bait-and-Switch: The Doctor assumes the Ice Warriors will be the Villain of the Week as usual. Turns out they've left their militaristic past behind them.
- Bait-and-Switch Gunshot
- Big Bad Duumvirate: Arcturus and Hepesh.
- Bookcase Passage: Just about any torch on the wall can be lowered to activate a secret door.
- Boy of the Week: There is definite romantic tension between King Peladon and Jo, despite him angering her with an overly-hasty proposal.
- Brains and Brawn: The Doctor and Grun.
- Brain in a Jar: Arcturus.
- Brief Accent Imitation: Jo starts speaking in the upper class variant of RP when the Doctor gets her to impersonate a queen.
- Contrived Coincidence: Jo had prettied herself up for a date with Captain Yates before she unwisely steps into the TARDIS, so she finds it easy to pass herself off as a princess.
- The fact that the TARDIS landed on Peladon at just the right time is at first seen to be this (as usual), but the Doctor hypothesizes it was actually the Time Lords.
- Continuity Nod: This isn't the first time the Doctor's impersonated an Earth examiner.
- Deadpan Snarker: Princess Jo complains of the incompetence of the pilot who caused them to crashland on this planet, while the Doctor rolls his eyes in the background.
- Duel to the Death: The Doctor has a choice: be executed for defilement of Aggedor's shrine or fight Grun to the death. He chooses the latter, but doesn't kill Grun when he wins.
- Does This Remind You of Anything?: As befits the Shakespearean setting of the Peladon stories, Alpha Centauri is named after the country (in this case, star system) it represents, rather than having a name of its own.
- The rude implications of its appearance are oft commented on by reviewers.
- Evil Chancellor: Well the chancellor isn't evil, but the High Priest fits the role.
- Falsely Reformed Villain: The Ice Warriors claim to want to have given up their militaristic ways, which the Doctor does not believe. In fact, they have.
- Fantasy Counterpart Culture: This story is an analogy of Britain joining the EEC.
- Fantastic Racism: A big part of the plot; Hepesh isn't just a stuffy conservative traditionalist, he's a racist, hateful towards the alien interlopers on his planet and contemptuous of his king's half-human blood. Also averted with the Galactic Federation; despite the treachery of Arcturus it's a pleasant surprise that the Ice Warriors turn out to be good guys, proof that even descendants of former enemies can become friends.
- The Federation: The Galactic Federation.
- Gentle Giant: Aggedor, once the Doctor calms him down.
- Half-Human Hybrid: King Peladon is half-Pel and half-human. His mother was a human princess. He later has a daughter, Queen Thalira. It's unknown who her mother was.
- Hard Head: Grun is merely unconscious for maybe a minute after he's bashed on the head with a rock.
- Hollywood Torches: So many the production team got in trouble for it, as the soot damaged the cameras.
- Homage: This story is one of times that Doctor Who has come closest to Star Trek, lampshaded by the "Federation" reference. In particular, it bears clear parallels with the Star Trek: The Original Series episode "Journey to Babel", with political intrigue among an assortment of distinctive-looking alien characters centring around a planet's application to join the Federation.
- I Owe You My Life: The Martian ambassador sides with the Doctor after the Doctor saves all delegates' lives.
- Lord Country: The ambassador to Alpha Centauri is named "Alpha Centauri".
- So is Peladon, king of Peladon.
- Mistaken for Special Guest: The Doctor is assumed to be the ambassador from Earth.
- Mistaken Identity: the Doctor is assumed to be the delegate from Earth.
- Music Soothes the Savage Beast: Aggedor is calmed by the Doctor singing a Venusian lullaby.
- Named After Their Planet: Alpha Centauri and Arcturus are named after the stars their home planets orbit.
- No-Sell: Grun isn't impressed when the soldiers punch him. A rock on the head, however...
- Not Always Evil: The Ice Warriors. Although they initially come across as potential villains, they turn out to have developed into a peaceloving race... although they still have weapons in case diplomacy doesn't work.
- Not So Extinct: Turns out there is a living member of Aggedor's species in the tunnels.
- Obscured Special Effects: Alpha Centauri'S Unfortunate Character Design was disguised at the last minute by putting him in a yellow cape. (It doesn't help.)
- Powered Armour: The Arcturus delegate is housed within a mechanical transport housing somewhat like a Dalek's, although in his case it's mainly for life-support in a non-aquatic environment.
- Proud Warrior Race Guy: Possibly Grun, definitely Izlyr (and presumably Ssorg).
- Punch! Punch! Punch! Uh Oh...: Happens when a soldier attempts to stop Grun, the King's Champion. He punches Grun several times to no effect.
- The Radio Dies First: Well, it's destroyed deliberately first.
- Real After All: Aggedor.
- Red Herring: Given his past difficulties with them, the Doctor naturally assumes the Ice Warriors are the bad guys.
- The Reveal: The Martians are the good guys here.
- Ripped from the Headlines: The story was written as a commentary on Britain entering the European Union.
- Running Gag / Title Drop: When the real delegate from Earth turns up at the end of the final episode."The Doctor? Doctor who?"
- Screw This, I'm Outta Here!: At the end the Doctor asks Jo if she wants to stay and become Queen, but then the real ambassador from Earth arrives and they beat a hasty retreat in the TARDIS.
- Ship Tease: Some definite tease between Jo and Peladon, undoubtedly Reality Subtext.
- The Speechless: Grun, a mute, and a reasonably heroic one once outmaneuvered by the Doctor.
- Ten Little Murder Victims: Various delegates from alien civilisations who have come to the castle of Peladon in a storm. The Ice Warrior delegate nearly dies in an accident and the delegate from Arcturus is found dying with a vital part of his life support missing. Everyone gets increasingly paranoid and existing racial tensions flare up.
- Took a Level in Kindness: When Jo ruins the Doctor's promising hypnosis of Aggedor, the Doctor, annoyed, tells her what he was doing, but forgives her a minute later and they move on. It mirrors their first meeting when she ruins his experiment and he berates and insults her, and shows how far their friendship has gone in the time they spent together.
- Well-Intentioned Extremist / Knight Templar: Hepesh. But we do learn eight Doctors later that the Strawman Has a Point, spelled O-O-D.
- Two stories later we learn something like this happened before.
- The X of Y