The one where Adric becomes an idiot.
Peter Davison's first filmed story as the Doctor (the episodes are often filmed out of order).note
In the first of his continuing attempts to bring Tegan back home, the Doctor gets the date and time exactly right... but is four light-days off, location-wise (so, somewhere in the Oort cloud). Oops. Instead, the Doctor and his companions land on a huge spaceship crewed by the delusional Monarch and his lackeys, the Ministers of Persuasion and Enlightenment. These three Urbankans, or frog-like aliens, travel in the company of four groups of humans: Ancient Greeks, Chinese, Mayan and Australian Aborigines.
Monarch wishes to go back in time before the Big Bang to watch himself creating the universe (he thinks he's God, you see), take over the Earth with those groups of humans in charge... and jack the TARDIS as his own ride for personal enjoyment (shooting for the moon, this one is). Adric shows excellent character judgment and begins sucking up to this raving mad frog alien while Nyssa is taken to be turned into an android (like every single other human who has been taken by the Urbankans, as it turns out). Adric also suddenly shows incredibly misogynistic behaviour, proclaiming that Tegan is "a woman" and wouldn't understand the complicated math behind anything, while simultaneously proclaiming Nyssa an exception because she's "a girl."
Meanwhile, Tegan, who was entrusted with the TARDIS key, ends up losing her cool and desperately attempts to hijack the TARDIS for a ride back to Earth. Completely breaking down emotionally, she actually persuades the ship to dematerialise, only for it to materialise floating in space outside Monarch's spaceship instead. Meanwhile, the Doctor is forced to watch a whole lot of tribal dancing, and sword-fighting, and half-naked men wrestling each other, and spends much of the episode rolling his eyes at everything.
In the end, the Doctor sets "young idiot" Adric straight and then goes flying around in space without a space suit (thanks to his Bizarre Alien Biology), but with the help of a cricket ball and The Power Of Dodgy Physics. He rescues the TARDIS, then randomly throws an incredibly deadly poison at Monarch, which doesn't kill him... but instead shrinks him and reveals that he is still part of 'flesh time' and was never fully processed into being a android. The robots announce that they're going off to find their own planet and "start again."
Everyone leaves, and Nyssa conveniently collapses at literally the last second because she originally wasn't written in the next episode. (In fact, if it wasn't for Davison's intervention, Nyssa would've been dropped as a companion altogether.) Her sudden collapse gets explained much later, in the Big Finish Doctor Who audio story "Primeval".
- Affably Evil: Monarch, who carries a pleasant and civilized demeanour (which, unfortunately, wins over Adric for most of the last two episodes).
- The Ark: The Monarch's ship contains his people, who left Urbanka when he destroyed it.
- Ancient Grome: Seriously, Greek gladiators? Somebody clearly did not do their research.
- Artistic Licence History: The Maya civilization being twice as old, or more, than it actually was.
- Artistic License Physics: In the spacewalk scene, conservation of momentum doesn't actually seem to exist. The Doctor runs into Space Friction on the way to the TARDIS and has to pull himself back to the airlock by the rope, and when the rope is untied he just sort of stops dead where he is. Throwing a cricket ball at the ship has no effect on him, but catching it on the rebound propels him backwards (as it should).
- Badass in a Nice Suit: Persuasion. Also has a wry sense of humour: On finding Nyssa using the Sonic Screwdriver together with pencil graphite to create a short circuit, confiscates the Sonic Screwdriver but adds:Persuasion: You may keep the pencil.
- Batman Can Breathe in Space / Space Is Cold: Wearing just an air-helmet, the Doctor explains that he can survive about five minutes in open space. Vacuum is apparently not a problem for his exposed skin; the time-limiting factor is explicitly named as the intense cold.
- Captain Obvious: A rather dark example as when Nyssa and Aldric go into an area without oxygen, and Enlightenment (who is watching the event on the monopticon alongside Monarch) notes that they have lungs. Monarch coldly replies that they should remember that.
- Characterization Marches On:
- Because this was Peter Davison's first filmed episode, and as such his characterization wasn't established, there are moments where he comes across as more short-tempered and snarky than usual.
- This is true of the other regulars as well, who all have noticeably less sympathetic characterisation - Adric is misogynistic, and elitist to the point that he approves of Monarch for a while; Nyssa is smug and so bland that it's impossible to tell when she's supposed to be drugged or hypnotised; and Tegan is bumbling and weepy. John Nathan-Turner said that this was the story in which the characters were written closest to what the season bible said, which makes it seem even odder.
- Continuity Nod: The Fourth Doctor previously displayed an ability to survive exposure to a vacuum in space in "Nightmare of Eden". The Eleventh Doctor would again demonstrate this ability in "The Doctor, the Widow and the Wardrobe"...and then the Twelfth would decidedly not demonstrate it in "Oxygen", just to muddy the waters.
- Desperate Object Catch: The Doctor saves his own life when he's adrift between two spaceships and nearly out of air. He tosses a cricket ball to bounce off one vessel, then catches it on the rebound, propelling himself towards the other with the cumulative acceleration of the throw and the catch. Had he missed, he'd likely have expired before reaching the airlock.
- Doomy Dooms of Doom
- Eternal Indigenous Australian / Suddenly Always Knew That: Good thing Tegan fluently speaks a 40,000-year-old dialect of a random Indigenous Australian language! (There are thousands of such languages, most of which wouldn't have existed by Tegan's time — and meanwhile, linguists in the rest of the world are having a tough enough time reconstructing Proto-Indo-European, which is a mere 10,000 years old.) Then again, the doctor specifically phrase the question as dialect than her knowing the language itself.
- And more importantly - why didn't the translation circuits on the TARDIS translate the speech for the Doctor?
- Everybody Lives: This one of the few stories with no deaths.
- Expospeak Gag: Adric displays both his scientific knowledge and personal immaturity to irritatingly ask for the "sodium chloride", when he really means the salt.
- A God Am I: Monarch, sans the actual godhood.
- Heart Drive: Used for a Cliffhanger when a supposedly human character opens himself up to reveal circuitry where flesh and bone should be.Bigon: This compound is not me. (takes out chip) This is me.
- High Collar of Doom: The Big Bad Monarch sports one, as can be seen in the page image.
- Horrible Judge of Character: Contrary to popular belief, this is the only story where Adric actually sides with the villain (his ploys in "State of Decay", and "Kinda" were attempts at being a Fake Defector that wound up falling flat, and in "Castrovalva" he was being made to work for the Master against his will), but only because he thinks that Monarch's intentions are legitimately benevolent.
- Hollywood Costuming: A production error in part two: one of the Greeks watching the Mayan dance from the balcony is wearing white lace-up trainers on his feet.
- Human Aliens: Lampshaded. Tegan mentions that she's human, and their alien hosts ask if the rest of them are too. The Doctor has to correct them that, no, he's from Gallifrey, Nyssa is from Traken and Adric is from Alzarius.
- Improvised Microgravity Maneuvering: The Doctor uses a cricket ball to propel himself from Monarch's ship to the TARDIS.
- Kill and Replace: Almost happens to Nyssa, who is placed in a machine to scan her to create a robot duplicate before disposing of the organic original, but she's rescued in time.
- Kneel Before Zod: The Doctor is forced to his knees, solely for the purpose of getting his head chopped off.
- Off with His Head!: The androids prepare to behead the Doctor at the end of Part 3.
- Out of Character: Matthew Waterhouse has said that just when he'd get a handle on what Adric was supposed to be like, the next script would play him differently. Particularly, he's a misogynistic Jerkass here and has his only true case of siding with the Big Bad of the serial of his own free will. (To be fair, the Monarch left the A God Am I and Take Over the World stuff out of his sales pitch.) The way he acts in this serial is unprecedented.
- Pun-Based Title: Monarch's spaceship is four light-days away from Earth, there are four ethnic groups on board, and the Doctor and his companions are four people. Or maybe the truth of the matter is that when this story was filmed (1981), the Doomsday Clock was positioned at four minutes until midnight.
- Real Life Writes the Plot: Nyssa faints at the end because Sarah Sutton's contract was being negotiated. It's also why she's largely absent from the next story.
- Ridiculously Human Robots
- Sci-Fi Writers Have No Sense of Scale: Monarch's ship is explicitly mentioned as not being able to travel faster than the speed of light and as having left Urbanka 1,250 years ago, placing Urbanka no more than 1,250 lightyears away. It's also mentioned as being in galaxy RA 1489note . The nearest galaxy to the Milkyway is 25,000 lightyears away.
- Slouch of Villainy: Monarch is content to let his ministers do all the bad stuff.
- Surveillance Drone: The Monopticon.
- Translation Convention: An ancient Australian Aboriginal language isn't translated, allowing Tegan to expand her character by being the only one able to understand and speak it. This is regardless of the fact that there are dozens of loosely related and unrelated Aboriginal languages, and the one they were speaking was unlikely to still exist by the time Tegan was even born. Then again, as mention above, it was less the language the doctor was surprise but the dialect.
- The Voiceless: Princess Villagra, who took a vow of silence.
- Wham Line: When the Doctor wonders how organic life survived for 35000 years on a spaceship.Bigon: The only organic life on this ship is in the floral chamber.
- What Is This Thing You Call "Love"?
- What the Hell, Hero?: Adric sides with Monarch for part of the story, for which the Doctor rightly calls him out. "Now listen to me, you young idiot—youre not so much gullible as idealistic. I suppose it comes from your deprived, delinquent background." It's especially epic coming from Five, who is less bombastic than most Doctors (although, as said earlier, this was his first filmed story and there's a good bit of Four left over before his characterization fully gels.)