The Doctor, Nyssa, Tegan and Turlough end up in the year 2329, when a comet passes through Earth's solar system. It passes by very quickly, and the Doctor is, oddly enough, really curious about the surface of it.
Oddly enough, the comet is a giant mall of sorts, and as they land and go their own merry ways, Nyssa and Tegan go shopping, the Doctor and Turlough are ... arrested.
Now the Doctor is forced to prove his innocence without any evidence, and Nyssa and Tegan are on the lam... with actual thieves. They are "Piggybackers", people who are allowed to be on the comet but not participate in the trading system.
"The Jupiter Conjunction" contains examples of the following tropes:
- American Accents: Falcao is played by a Brit. And doesn't sport a very convincing one.
- Arson, Murder, and Jaywalking: Turlough's reasoning why the Jovians are attacking human beings on the comet.Turlough: Maybe they're dying out!? Maybe they need a new planet!? Maybe .. their dog died and they're upset about it..!?
- Blatant Lies:Nyssa: You thought I died?Tegan: Never. Not for a second.
- Call-Back: Nyssa has symptoms of not being fatigued because she got rejuvenated from the previous episode The Emerald Tiger. The Doctor likens it to his regeneration from his third to fourth incarnation.
- The Doctor promises Nyssa, as Stuff Is Blowing Up on the comet, and the Doomsday Device is about to fire, and the Jovians are about to die, and Earth runs risk of dying, that he'll track the comet and find her. When they stop communication, she mutters to herself..
Nyssa: You once said much the same thing to someone else...- When Nyssa seems to have died the Doctor muses the same thing. Tegan tells him that Nyssa's death wouldn't just weigh upon him, but three other people note , but she falls short of telling him when the TARDIS vwoarps in.
- When Major Nash is about to let himself and Nyssa be consumed by a massive explosion, Nyssa implores him to let her live because he doesn't know what important work he'll destroy if he lets her die.
- The Hostile Action Displacement System saves Nyssa in the end.
- The Doctor and Turlough use the TARDIS atmospheric suits.
- Catchphrase: The Fifth Doctor's "Brave heart" is used, during Nyssa's Disney Death.
- Death Equals Redemption: The Jovians, after the Particle Cannon has been set up to fire and the timer can't be stopped. They aren't proud of their actions, but they can at least trigger the explosion to prevent the cannon from firing at Earth. They all die, save for one.
- Doomsday Device: The Jupiter Particle Cannon, developed by the Jovians and the Jupiter Axis, can destroy most of Earth.
- Driven to Suicide: Major Nash tries a Taking You with Me when he realizes he's been lied to, and used as a scapegoat, in a war he thought was about honour.
- Fall Guy: The Jovians have been set up by Jupiter Axis Major Nashnote to appear as a common foe to band against, with Earth forces.
- Fantastic Racism: Between "Earthers" and non-"Earther" humans, and so called "Piggybackers" people who are outside of the barter system on the comet.
- Fountain of Youth: Nyssa looks younger because of events in the previous adventure.
- Genghis Gambit: Jupiter Axis Major Nash has the Jovians set up a Doomsday weapon so that when they attack, per the Jupiter Axis' orders, Earth would join with the Axis against them (the Jovians).
- The Guards Must Be Crazy:Turlough: Would you like to do the honours?The Doctor: It'd be my pleasure. AAAH! AARGH!Turlough: Guard! Guard! There's something wrong with the prisoner!
- Heel–Face Revolving Door: Turlough appears to betray the Doctor at the end of part 1, and is convincing enough that the Doctor actually buys it for some time. As it turns out, it was all part of The Plan.
- Honor Before Reason: Major Nash tries to fire the Particle Weapon before he dies, after his entire life's work crumbles around him, literally and figuratively.
- Indy Ploy: Turlough stages a kidnapping on the fly when confronted with security guards.
- Just Between You and Me: Turlough actually does this with one of the security personnel, to prove his innocence in murder.
- Kangaroo Court: The Doctor and Turlough are part of one. Turlough is able to exploit it in order to engineer a later escape attempt.
- MacGyvering: Nyssa makes a defibrillator out of a Neural Scrambler.
- The Man Behind the Monsters: The Jovians aren't the ones enacting the plan and setting up a Doomsday Device, it's Jupiter Major Nash, Earth forces.
- The Man Behind the Man: Major Nash himself is being played by Violet, and he doesn't know about the peace treaty between the Jupiter Axis and Earth Forces. He legitimately believes he's following orders for a war four years in the making.
- Mistaken For Thieves: The Doctor and Turlough are immediately arrested when they appear with a ship that can materialize inside closed areas. Since the recent theft of one-third of the cargo remains unsolved. Ah.
- The Mole: Patricia Walton works for Major Nash, and sets up the false footage, and heads the Kangaroo Court.
- Nice Job Breaking It, Hero: The Doctor decodes the scrambled security footage... which shows the TARDIS stealing the cargo.note
- Rogue Agent: An Invoked Trope by Violet, who says that if Earth finds out during the peace conference that the Jupiter Axis was planning to stage an attack from Starfish Aliens Earth didn't know about, they'll go with the "rogue officer" excuse, and use Major Nash as a Sacrificial Lion.
- Shock and Awe: The Jovians can fire lightning bolts from their cloud bodies, and need about 40 seconds to recharge before they can fire again.
- Starfish Aliens:Manny: Creatures... made of clouds!!
- Stuff Blowing Up: The entire comet ends up being blown to smithereens because of a secret Doomsday Device.
- Uncomfortable Elevator Moment: When Falcao takes the Doctor and Turlough prisoner, they get stuck in an elevator with horrible muzak playing.
- Xanatos Gambit: While confessing to "his crimes", Turlough sends the security force on a wild goose chase all over the comet, while he gets put back into custody with the Doctor, making it easier to escape.
