The one with ping-pong eyeballs.
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 travellers 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.
The Monoids plan to colonise Refusis II themselves, but the invisible Refusian natives help the Doctor persuade both races to live in peace.
- Aliens Are Bastards: The Monoids, though they were only treating the humans like they had been treated.
- Awesome Anachronistic Apparel: Dodo raids the TARDIS wardrobe (without permission, much to the Doctor's irritation) before her first adventure, set aboard a Generation Ship around the time of the sun going supernova. She spends the whole story wearing Ian's Knight of Jaffa tabard, which both the Doctor and Steven criticise her for - Steven's reasons being because it's inappropriate, and the Doctor's being in case she ruins it and they need it later. In her case, it's less to emphasise her coolness and more to emphasise just how utterly weird she is.
- The Ark: Duh
- 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.
- Continuity Cavalcade: When questioned by the humans as to things he has seen on his travels, the Doctor seemingly mentions events from "The Romans" and "The Myth Makers" and mentions the Daleks. All of this took place in what the humans now call the first segment of time.
- 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.
- Definite Article Title: This serial, and all four episodes.
- Deus ex Machina: How the Refusians deal with a nuclear bomb.
- Disproportionate Retribution: The Monoids are treated as second-class citizens early on, but they're not exactly oppressed. That doesn't stop them oppressing the humans when the tables are turned.
- 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 rematerialise exactly 700 years after they left.
- Food Pills: The Ark's food pills aren't for eating whole, but a clever storage solution. Drop one in water, and you get a proper meal.
- Generation Ships: The title spacecraft serves as both a generational ship for its crew (of both humans and subservient aliens), and as a Human Popsicle-stand for the remaining billions of humans and subservient aliens, since a ship that could carry both species in their entirety would've been far too massive to build (or move).
- 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.
- Nice Job Breaking It, Hero!: The Doctor arrives on board a generation ship, unwittingly exposing everyone within to the common cold. This turns into a plague due to their lack of resistance and many people die until the Doctor can engineer a cure. This would be bad enough if a TARDIS glitch didn't cause him to accidentally travel hundreds of years further in time to the future, revealing that the plague had created conditions for the humans' slave race to rebel and enslave the humans.
- Noah's Story Arc: Well, the clue is in the title.
- Off-the-Shelf FX: The Monoids' single eyes were actually ping-pong balls.
- Ooh, Me Accent's Slipping: There are occasional lapses by Jackie Lane into the Cockney accent in which she rehearsed the story before the production team were instructed by their superiors that it was unacceptable for a regular character to speak in anything other than "BBC English".
- Relocating the Explosion
- The Revolution Will Not Be Civilised
- Rummage Sale Reject: Those streamer outfits. Well, at least they tried something new...
- Slave Race: Two of them.
- Sleeping Dummy: Venussa and Dassuk pull this trick to allow Dassuk to escape from the Security Kitchen and free the slaves.
- 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.
- Your Princess Is in Another Castle!: Episode 2 ends with the Doctor curing the plague that had infected the Ark and departing in the TARDIS. Then they rematerialise back in the same spot, except now the statue that was meant to take 700 years to complete is finished...and has a Monoid's head.