"The Sisterhood are up to something. Remember that Old Earth saying? Never trust a nun, never trust a nurse, and never trust a cat."The Tenth Doctor, mostly recovered from his regeneration trauma, heads off for more adventures! He takes Rose to visit New New New New New New New New New New New New New New New New New York, not only to continue showing her the universe but also because he got a message from there on his psychic paper. Rose is enjoying her bouncy, chatty "New New Doctor".The message leads the Doctor to a hospital run by cat nuns but that's not even the strange part. There's a guy who's turning into stone, but even that isn't the strange part. Among all the cat women and the people Taken for Granite, what's peculiar is that the hospital is too far ahead of its time: they have cures for diseases that haven't been cured yet.The Doctor is on the case! In the ward, he and Rose meet the apparently dying Face of Boe, the oldest being in the galaxy. Face is dying of old age, which is the only thing the cats cannot cure.Rose, meanwhile, meets Lady Cassandra (remember her?), who wants her body. Literally. She snatches Rose's body and mind, and quickly realizes that a) Rose likes the Doctor! b) the Doctor is hot! c) a good old-fashioned makeout session is called for, and d) Rose is a chav! The Doctor doesn't mind at all.So the Doctor and a Cassandra-possessed Rose stumble upon the source of the cats' medicines — a secret colony of artificially grown people, who are held prisoner and infected with everything in order to manufacture cures for everything. The clone-people get out, the Doctor figures out that Rose is not quite herself, and Cassandra is forced to switch back and forth between the bodies of the Doctor and Rose during the ensuing chase. While David Tennant camps it up as Cassandra (and Billie Piper tries very hard to keep a straight face), Cassandra at one point ends up in the body and mind of one of the sick, and has a massive Villainous B.S.O.D. when she feels what it's like to really suffer.The Doctor gets baptized by the nuns with a panacea cocktail and, with a touch, disinfects the clone-people in a scene that would make Jesus Christ himself jealous. It's not the last time the Doctor will be a Messianic Archetype on this specific planet. Cassandra goes into the body of her dying servant, where she finally dies in her own arms through a bit of foreshadowed time-travel. The Face of Boe, meanwhile, decides that he could stick around for a bit longer after all, and promises to get back to the Doctor later on that big-important-secret-to-be-imparted-to-the-Doctor-at-the-time-of-the-Face's-death thing.
- Alas, Poor Villain: Despite her having soared over the Moral Event Horizon long before she even appeared, Lady Cassandra gets a surprisingly well-written death scene. Becomes an actual Tear Jerker when you realize the Fridge Brilliance behind it: when her older self expired before her, she was the only person trying to save him/her (though some did walk off, presumably to get help while everyone else stood around apathetically), shouting for people to try and help only for people to back away from her and not do anything. Not only did this tarnish her reputation as an upper-class lady (nobody ever called her beautiful again after Chip), but it was the start of her conviction that people could only rely on themselves. She finally realized that she was wrong about this when the Doctor commended her on helping to cure those artifically created humans, and it was this which helped her accept her long-delayed death. Not to mention that despite her snobbery, she never did forget that Chip's face was that of the last person to ever compliment her.
- Berserk Button: The Doctor twice:
- He's absolutely livid at the way the Sisterhood uses its clones (ie. living things) as essentially husks for diseases.
- He's not too happy about Cassandra's Grand Theft Me, especially when via Rose, quite convinced she should accept her time. When left with the problem of her finding a willing host, he still pleads her to see reason, exclaiming such a fate just plain isn't fair.
- Blackmail: Attempted by Cassandra against the Matron: "Give me your donations or I'll tell everyone about your human-sized petri dishes."
- Body Snatcher: Cassandra's Evil Plan is to use a psychografting machine to take over Rose's body, intending to live on by using Rose's body as a replacement for her dying one.
- Brain in a Jar: How Cassandra survived her apparent death at the end of her previous appearance; her skin may have torn itself apart, but her brain remained intact, meaning that her mind survived just fine, apart from being left severely pissed-off at the Doctor and Rose.
- Camp Gay: Cassandra in Chip's body has the mannerisms and Cassandra in the Doctor's body arguably more so. Chip, even when unpossessed, has a lisp and spring to his step.
- Cat Folk: The Sisterhood are all humanoid cats.
- Chekhov's Gun: The awkward decontamination lifts, best described as a human car wash, make a good dispersal system for the Doctor's cure cocktail.
- Corpsing: Billie Piper makes a valiant effort to hide her amusement during the "Goodness me, I'm a man!" scene.
- Clones Are People Too: The Doctor is furious at the way the Sisterhood treats the clone-people. He sees them as just alive as himself or Rose.
- Curse Cut Short:
Lady Cassandra: That piece of skin was taken from the front of my body. This piece is the back.
- To begin:Lady Cassandra: At last, I can be revenged on that little—
(cut to Rose and the Doctor)
Rose: Bit rich, coming from you.
Rose: (laughing) Right, so you're talking out your—
Cassandra: Ask. Not.
- To begin:
- Decontamination Chamber: Set up as a Chekhov's Gun and used in the climax to disinfect all the test subjects of their illnesses.
- Died in Your Arms Tonight: Cassandra, with her younger self.
- Eating the Eye Candy:
Cassandra: Ooh, he's slim... and a little bit foxy. You thought so too. I've been inside your head. You've been looking...you like it.
- What Rose has been doing, according to Cassandra. The smile Rose gives when Cassandra accuses her of such confirms it.
- Cassandra herself — five minutes after being dumped into a twenty-something body full of hormones she can't resist grabbing the Doctor for a snog, even though she hates the Doctor and wants to kill him nastily.
- Evil Is Hammy: Billie Piper and David Tennant have a grand old time camping it up as Cassandra. In fact, Russell T Davies says this came about because Billie Piper wanted at least one episode where she got to be funny.
- Evolutionary Stasis: Despite being said to have changed, the humans still look human. Or, according to the Doctor, they look human again.
- Fantastic Racism: Cassandra (once again) dismisses humanity's other descendants as mutants unworthy of the name human. Rose retorts that they just evolved, while Cassandra has mutilated herself rather than move with the times.
- Face Death with Dignity: Cassandra, as much as she can in her situation anyway.
- Flying Car: Seen in establishing shots for the "this is the future" feel.
- Gender Bender: This is happening when Cassandra is possessing the Doctor's body. David Tennant appears to be having a ball acting like he's possessed by a woman.
- A God Am I:
- "I am the Doctor. If you don't like it, if you want to take it to a higher authority, there isn't one. It stops with me."
- Averted, believed it or not, by Cassandra. Despite having someone who explicitly "worships her", she doesn't think of herself as a god.
- Grand Theft Me: Cassandra does this with a lot of people: Rose, The Doctor, one of the clones and Chip, but Chip doesn't count. He explicitly volunteered for it.
- Karmic Death: Both cat nuns who callously executed a clone man begging for help are later killed by the diseases carried by the clone people.
- Lower-Class Lout: Cassandra invokes the "snob calling someone a chav" version towards Rose while engaging in Grand Theft Me.
- Magic Plastic Surgery: Cassandra seems to think the Doctor's new face is due to this; hence why she calls him a hypocrite for scorning her own surgery.
- Man, I Feel Like a Woman: Cassandra spends the episode jumping around being Billie Piper, David Tennant and a couple of other people. "Goodness me, I'm a man!"
- Messianic Archetype: The Doctor calls all the disease-ridden clone-people to him so he can cure them with a sprinkler (baptism) and then sends them out to cure others by laying on hands like the apostles.
- Not Herself: Rose, after being possessed by Cassandra. The Doctor can tell because Rose would care about the people locked up for cures and Cassandra does not.
- Opinion Flipflop: Used lightly by the Doctor. When attempting to reason Cassandra out of Rose's body (and thus to her inevitable death), he insists that there is no other way and her time is over. When she instead uses Chip as a host (something the Doctor cannot forbid as he is a volunteer) he quickly relents and offers to keep her alive on a skin graft if she leaves his body.
- Our Zombies Are Different: The clone-people are stated to be "not proper life forms" and have the "zombie gait" and moans.
- People Farms: The clone-people are grown in test tubes by the Sisterhood to assist in their cure research.
- People Jars: Cassandra does this to a lot of people; at this point she sees them as vessels.
- Post-Kiss Catatonia: The Doctor after Cassandra-in-Rose's-body snogs him.The Doctor: (squeaky) Yep. Still got it.
- Powered by a Forsaken Child: The Sisterhood's fast-acting medicine is powered by a veritable plethora of clones infected with everything.
- Reverse Psychology: A masterful example from Cassandra. She tells Rose that the cat nurses have been keeping a secret, and asks her to come in close so she can whisper it in her ear. Rose laughs at such an obvious trap and instead takes a step back and ends up immobilised by the Psychograph.
- Saintly Church: The Sisterhood run a hospital founded by charity to heal the sick. They take oaths to do nothing but heal the sick. One of them says that the "people farm" thing was a last resort because nothing else they tried worked fast enough to heal all the sick people coming to them. When Cassandra releases the clone-humans, they quarantine the hospital to prevent anyone else from getting sick. They are still in the hospital when this happens.
- Showing Off the New Body: Cassandra does it as soon as she takes over Rose:Cassandra-in-Rose: Look at me! From class to brass! Although... (zips her jacket down and runs her hands over her body) Oh... curves... oh, baby... (bounces up and down) It's like living inside a bouncy castle!
- Spot the Imposter: Cassandra's ridiculous attempt at 21st century Earth English while in Rose's body, coupled with a knowledge of New Earth computer systems, gives her away early. You might also add her passionate snogging of the Doctor, but he seems to put that down to his own irresistible charm.
- Stable Time Loop: Cassandra says Chip was made in the image of her favorite pattern — which came from the last person to tell her she was beautiful, inspiring said pattern. That last person is Cassandra herself in the pattern's body, by the by, who also inadvertently starts Cassandra's murderous vanity.
- Still Got It: The Doctor believes Rose-Cassandra's random snogging him is due to his irresistible charm, which, naturally, carried over from his regeneration.
- Suicide Dare: Not so much dare as attempted order. Both Rose and the Doctor take turns bellowing at Cassandra to get out of the others' body (and thus ultimately die without a host). They can't seem to fathom why it doesn't work. After left with the dilemma of her finding a willing host, the Doctor attempts to haggle by offering to keep her alive on a skin graft and take her to be tried for her crimes.
- Sycophantic Servant: Chip, despite being once left for dead rather apathetically by Cassandra, dotingly serves her and cheerfully donates his body as a host (and thus facilitates his death), even scolding the Doctor for objecting.
- Taken for Granite: One of the diseases causes this.
- Touch of Death: The clone-people are infected with every disease known to the galaxy; some of them can kill in minutes. All of it is infectious. The Doctor inverts this trope by curing a bunch of them and sending them out to cure the rest by touching them.
- Undying Loyalty: Chip is extremely faithful to his mistress Cassandra. How faithful? He volunteered for Grand Theft Me, even if it means dying for her.
- Victoria's Secret Compartment: Pulled by Cassandra in Rose's body for a container of knockout gas.
- Villainous B.S.O.D.: Cassandra, after ending up in the body of one of the sick, discovers what it was like to be one of them.
- Well-Intentioned Extremist: The nuns, who use thousands and thousands of scratch-made humans and infect them with every disease known in the galaxy in order to find effective and fast cures for diseases, where cures would normally take thousands of years to find. The episode itself plays with this, as one head nun callously executes a test subject who had gained sentience and another seems more angry at the Doctor for ruining the Sisters' reputation, as well as cursing the humans for bringing so much illness with them. The survivor is more upset about the people that will be infected if the clone-people escape, albeit because she (and the rest of the nuns) don't think the clones are actual people.
- We Will Have Perfect Health in the Future: Discussed.Rose: I thought this far in the future, they'd have cured everything.
The Doctor: The human race moves on, but so do the viruses. It's an ongoing war.
- Woobie, Destroyer of Worlds: The clone-people are walking plagues but all they want is for someone to help them and love them.