[[caption-width-right:350:Get ready, boys and girls; the Doctor's about to monologue!]]

->''Homo sapiens. What an inventive, invincible species. It's only a few million years since they crawled up out of the mud and learned to walk. Puny, defenceless bipeds. They've survived flood, famine and plague. They've survived cosmic wars and holocausts. And now, here they are, out among the stars, waiting to begin a new life. Ready to outsit eternity. They're indomitable. Indomitable.''
-->-- '''The Doctor'''

Before ''Film/{{Alien}}'', there was "The Ark in Space".

The TARDIS arrives on an apparently deserted space station in Earth orbit, some time in the far future. Investigating, the Doctor, Sarah and Harry discover that the remnants of the human race are in suspended animation awaiting Earth's return to habitability following solar flares. The Doctor realises that the Earth is in fact now habitable again and activates the controls to begin reviving the humans. However, there appears to be something amiss -- some controls are damaged (which is why the alarm clock didn't go off) and there are green trails around the place.

It seems that a race of giant insects -- the Wirrn -- are also aboard. Eggs have been laid in the solar stacks, and larvae are glimpsed. One of them infects the human leader, Noah, who begins turning into a Wirrn himself. The Doctor and his companions gain the trust of the other defrosted humans, now led by medical officer Vira, and together they lure the hatched Wirrn onto a shuttle craft and launch it into space. Noah, using the last vestiges of his humanity, deliberately doesn't set the shuttle's stabilisers and it explodes.

Both Creator/RussellTDavies and Creator/StevenMoffat have cited this as their favourite serial from the original run of ''Doctor Who''.
* AirVentPassageway: Sarah Jane is the only one thin enough to get through the vent.
* AlmostOutOfOxygen: One of the first things the Doctor needs to fix on the Ark is the oxygen supply. Later on, the Wirrn deliberately cut off the air, because they can survive for longer periods without breathing, but the humans obviously can't.
* ApocalypseHow: Solar flares toasted the Earth, likely afflicting Total Extinction on anything that hadn't evacuated.
* TheArk
* BigCreepyCrawlies
* BodyHorror: Noah's transformation.
* ButtMonkey: Harry Sullivan. Almost everything the Doctor says to him in this episode, even when he's complimenting him on figuring something out for himself, amounts to "See, Harry? You're not a ''total'' fucking idiot, are you?"
* ChekhovsGun: While working on the shuttle craft, the Doctor removes a piece of metal and puts it in his pocket. It's still there in the next story and saves his life when the bad guy shoots him.
* CryonicsFailure: Because the Wirrn sabotaged it, the waking system doesn't work. And Dune is eaten by Wirrn larvae while suspended.
* DeadpanSnarker: Rogin.
* DecontaminationChamber
* EarthThatWas: A marginal case, since the whole point of the Ark was to enable humans to repopulate the Earth when it was safe to do so. So really it's the Earth That Was And Will Be Again.
* EmotionlessGirl: Vira never shows much emotion, at least until the moment she realises the mission has succeeded..
* FightingFromTheInside: Noah manages to get a message back to Vira while the Wirrn is converting him. And at the end, he is strong enough to not save the Wirrn despite knowing how.
* FlingALightIntoTheFuture: The purpose of the Ark is to make the human race survive.
* HeroicSacrifice: Both Rogin and Noah.
* HighVoltageDeath: The Wirrn's main weakness.
* HumansAreTheRealMonsters: In spite of the Doctor's obvious faith in humanity, there is evidence that they are not so virtuous after all. The Wirrn's plan to destroy humanity and steal their knowledge is an act of revenge for the destruction of their original breeding colonies in Andromeda by the human empire. When Vira learns of this, she is delighted that the occupation of Andromeda was successful, instead of sad that it caused large scale misery for another species.
* HumansAreSpecial: See the page quote.
--->"It may be irrational of me, but humans are quite my favorite species."
* IKnowYoureInThereSomewhereFight: The Doctor appeals to what is left of Noah's humanity by reminding him of the beauty of Earth. Noah claims to have no memories of it, but considering how he stops the Wirrn at the end, it seems the Doctor got through to him.
* InspirationalInsult: The Doctor intentionally insults Sarah Jane and hurts her feelings in order to help her achieve the AirVentPassageway escape. Needless to say, he doesn't mean it.
* LawfulStupid: Every human in this time, apparently. Noah is the worst.
* LightningGun: The automatic defense system has one.
* MoreExpendableThanYou: Rogin to the Doctor
* NotNowKiddo: Done to Sarah Jane. ''Twice.'' Eventually, the Doctor reveals he's doing this as a SecretTestOfCharacter, trying to make Sarah feel undervalued so she will rise to the challenge. In truth, the Doctor is very proud of her.
* NotThatKindOfDoctor: The Doctor claims that his is honorary, and that Sullivan can only treat sailors.
* PeekABooCorpse: Done as a CliffHanger with a Wirrn corpse rather than a human one.
* SleeperStarship: Well, it's meant to be gone for 5 millenia.
* TheVirus
* YouAreInCommandNow: Noah appoints Vira as the commander when he realises he's no longer able to.