All colonists are eccentric. That's why they're colonists.
— Captain Dent
The Master is at it again - this time he's stolen the Time Lords' files on The Doomsday Weapon, and they decide to let the Doctor off his leash a bit so that he can go and retrieve the files for them. The Doctor ends up on the desolate planet of Uxaerius, in the year 2472, where a small colony and an orbiting ship from the Intergalactic Mining Corporation are arguing about who owns the planet. Meanwhile the primitive natives are worshipping a strange machine guarded by a fearsome creature.
The Master shows up, posing as an Earth Adjudicator there to decide the fate of the planet. He forces the Doctor to take him to the Primitives' underground city, and it transpires that the strange machine is, inevitably, the Doomsday Weapon itself. Capable of destroying whole planets when activated, even when dormant its radiation has killed most of the race of the machine's guardian, as well as causing the colonists' crops to fail.
The Doctor persuades the last Guardian to destroy the machine rather than let the Master get his hands on it, and the Doctor and Master get away just as it explodes. The Master escapes in his TARDIS while the Doctor is sent back to Earth in his literally one second after the Brigadier demanded that he'd return
- Actor Allusion: the Brigadier tells the Doctor he'd nearly arrested the Spanish ambassador, mistaking him for the Master. The actor who played the Master, Roger Delgado, had previously played Mendoza, the Spanish envoy to the court of Elizabeth I in Sir Francis Drake.
- Affectionate Gesture to the Head: The Doctor gives Jo a kiss (something he rarely gives a companion, let alone anyone else) on the forehead after finding her safe and sound inside the Uxariean city. Both Jon Pertwee and Katy Manning seem to fumble awkwardly◊ in the lead-up to it.
- Alternate Landmark History: On a cosmic scale. The Crab Nebula was created during the testing of the Doomsday Weapon.
- Artistic License - Astronomy: The Sun is way too small to go supernova. Maybe not if it's blown up by some superweapon, but the Doctor and the Master talk about it eventually exploding in 10 billion years even without the weapon.
- BBC Quarry
- Big Bad Ensemble: Dent is the main villain of the story though the Master is naturally plotting to take over the universe as usual. Although they help each other out in the trial they have conflicting agendas, with the Master even offering to help the colonists.
- Brick Joke: One of the best in the series.
Brigadier: Doctor, come back at once.
- Corrupt Corporate Executive: Captain Dent.
- Crapsack World: Why do the colonists cling to a barren desert planet with barely enough food to support them? Because Earth has degenerated into an overcrowded, war ravaged nightmare while corrupt corporations strip every planet in their path and have complete power over most of the galaxy's citizens. Hard to blame the Doctor for looking so tempted by the Master's offer.
- Crystal Spires and Togas: The Time Lord council are shown wearing the latest Roman fashions.
- Doomsday Device
- Evil Cannot Comprehend Good: The Master can't seem to grasp the fact that the Doctor is perfectly content with just visiting planets, rather than wanting to rule them.
- Exactly What It Says on the Tin
- Heroic Sacrifice: Ashe
- Hive Caste System: the Uxarian warriors, priests and leader.
- Impersonation Gambit: The Master, yet again.
- Kangaroo Court: Unusual, in that it comes to the right decision.
- Knockout Gas
- The Master
- Minion with an F in Evil: Caldwell
- The Mole
- No Ontological Inertia: the ending implies that without the radiation created by the Doomsday Weapon the planet will instantly become fertile.
- Omnidisciplinary Scientist: When asked if he's "some kind of scientist", the Doctor replies that "I'm every kind of scientist."
- Omniscient Council of Vagueness: The Time Lords.
- Planet Looters: IMC.
- Scooby-Doo Hoax
- Slurpasaur: Subverted. The giant lizard that just looks like a blown up image of an iguana turns out to be...a blown up image of an iguana projected over a robot with claws, to simulate a giant lizard attack.
- Space Western: You've got the Determined Homesteaders trying to make a go of it in an inhospitable environment (the colonists), the greedy mining company who wants to take their land (IMC), the Circuit Judge who's the only law in the area (the Adjudicator), and the Magical Native Uxaerians. The Doctor himself is of course The Drifter.
- The Speechless: the Primitives and their priests. Their leader is the only one that can talk.
- Star Killing: The Doomsday Weapon is capable of this, though it's not used during this story.
- Token Good Teammate: Caldwell, who is all for driving the colonists off of the planet, but stops short at murder.
- Villain Exit Stage Left: While everyone else is distracted by the final confrontation of IMC and the colonists, the Master steals a car and returns to his TARDIS.
- We Can Rule Together: The Master offers the Doctor a chance to rule the Universe together. The Doctor refuses.
The Doctor: I want to see the universe, not rule it!
- What Happened to the Mouse?: One of the biggest in Doctor Who history: Captain Dent, the Big Bad of at least half the story, disappears completely midway through the last episode and it's never revealed what happened to him after his troops are defeated.
- Zeerust: The colonists still use typewriters.