"Take them to the security kitchen!"The eponymous Ark is a giant spaceship carrying Earth's surviving human, plant and animal life in suspended animation en route to a new planet, Refusis II. A few human guardians and their one-eyed, mop-topped servants the Monoids take care of the ship, but when the travelers arrive a plague quickly spreads. Dodo has a cold, and the common cold was wiped out a long time ago and neither human nor Monoid has any resistance.The travellers are placed on trial for deliberately spreading the plague, but the Doctor manages to find a cure and they are allowed to leave......only to rematerialise on board the Ark as it nears the end of its voyage. Since the plague, the Monoids have grown stronger and it is the humans who are the slave race.Oops.The Monoids plan to colonise Refusis II themselves, but the invisible Refusian natives help the Doctor persuade both races to live in peace.Watch it here.
— Monoid One (spotting a shortcoming in the Ark's design — right next to the laundry brig)
- Aliens Are Bastards: The Monoids, though it's strongly implied to be the humans' fault.
- The Ark
- Bad Liar: The Monoid who gives the whole game away to Dodo.Dodo: You'd better hurry up and get everyone down from the Ark. With all those people it's going to take awhile.Monoid: (chuckles) Don't worry, it might not take as long as you think!Dodo: What do you mean, are you up to something?Monoid: (glances nervously at the human slave behind him) N...n..no.
- Breather Episode: In comparison to the previous two stories, "The Ark" is much more lighthearted.
- Coconut Superpowers: The Refusians, being invisible, save money on rubber suits.
- Deadpan Snarker: At the end of episode 3, after the landing pod has been blown-up, and the Doctor and Dodo are trapped the surface, Dodo asks what they'll do if the Monoids decide to leave Refusis and find another world. The Doctor replies:Doctor: Well, in that case, we shall just have to stay here.
- Deus ex Machina: How the Refusians deal with a nuclear bomb.
- Earth That Was
- Enemy Civil War: The Monoids' violent disagreement on whether to colonise Refusis
- Exty Years from Now: In-universe. The Doctor and companions rematerialize exactly 700 years after they left.
- Generation Ships
- Halfway Plot Switch: Comes close to being two stories in the same setting.
- He Who Fights Monsters: After freeing themselves from humans, Monoids became worse than them.
- Human Popsicle
- Made a Slave: The transition between the two slave races
- Monochrome Casting: All of the humans aboard the Ark appear to be white. "The Beast Below", with its multiple spaceships from different nations, may be an Author's Saving Throw attempt to remedy this.
- Monument of Humiliation and Defeat: When the Monoids take over they turn the inspirational statue into a depiction of a Monoid instead of a human.
- Relocating the Explosion
- The Revolution Will Not Be Civilised
- Slave Race: two of them
- Space Clothes: The Guardians (see above)
- Starship Luxurious
- We Will Have Perfect Health in the Future: Played straight and subverted. The Guardians have cured the common cold long ago. Unfortunately this means they have no resistance to it.
- We Will Use Manual Labor in the Future
- You Are Number 6: The Monoids after their revolt, seemingly voluntarily