The one where the Doctor takes Rose to a hospital run by cats, Cassandra crashes the party by body-swapping, and everyone gets a free sample of deadly plague before being miraculously cured by some buckets of disinfectant.
Short Synopsis
The Doctor brings Rose to New Earth in the year 5,000,000,023, where a futuristic hospital run by cat-nuns hides a dirty secret: they’re growing human clones as disease carriers. Cassandra, the last “pure” human, returns and starts body-hopping — including into Rose — before learning the hard way that you can’t Botox your way out of mortality.
Extended Storyline
The Doctor decides to show off his brand-new self by taking Rose to the far future. Destination: New Earth! Which, despite being in the year five million and change, looks suspiciously like a shiny bit of Cardiff with a CGI facelift. Still, it’s got a hospital, it’s got cat nurses, and it’s got enough mystery to keep things interesting.
Rose is thrilled… until she meets Cassandra. Yes, that Cassandra — the stretched-out piece of human skin with a face we last saw exploding in The End of the World. Turns out she didn’t quite pop — she had a brain and some organs stashed away. (Because apparently if you’ve got a spare pancreas in a jar, you’re immortal.) Now with her loyal servant Chip, she’s determined to live forever. Again.
The hospital itself has problems bigger than Cassandra. The Sisters of Plenitude (the aforementioned cat nurses, looking like someone mashed Cats with Grey’s Anatomy) are running experiments. They’ve grown thousands of human “flesh” clones and infected them with every disease in the galaxy. It’s all very hush-hush, but the result is miracle cures for paying customers. Five-star Yelp reviews, questionable ethics.
Cassandra, being Cassandra, decides this is the perfect backdrop for her comeback tour. Using a handy body-swap gizmo, she jumps into Rose’s body. Cue Billie Piper suddenly swanning around with Cassandra’s fabulous diva persona — all flirty struts, arched eyebrows, and “Oh my god, I’m a chav!” lines. It’s both hilarious and deeply uncomfortable.
The Doctor, initially bemused, quickly realizes Rose isn’t herself. Unfortunately, Cassandra isn’t picky — she hops into the Doctor too, giving Tennant a chance to camp it up in style. Watching him flick his hair and preen like a catwalk model is worth the price of admission alone.
Meanwhile, the clones — the poor souls bred to suffer every disease — break out of their pods. They’re covered in boils, coughing up all manner of fluids, and desperate for contact. It’s 28 Days Later meets Scrubs. The Doctor realizes that they’re not monsters — just unlucky humans who want affection. But to everyone else, it looks like a zombie apocalypse in a hospital corridor.
The Doctor, now back in control of his body, does what he does best: improvises a cure. He mixes together all the hospital’s miracle medicines and sprays the infected patients down with buckets of the stuff. In five minutes, everyone’s healed, the crisis is over, and the Sisters of Plenitude are under arrest. Who needs years of clinical trials when you’ve got Tennant with a spray bottle?
As for Cassandra, she’s finally forced to face the inevitable: death. In a surprisingly poignant twist, she ends up body-hopping into her loyal servant Chip — only to discover that Chip was grown from a copy of her younger self’s DNA. With no more strength left, Cassandra dies in her own arms, comforted by her past self. It’s one of those moments where Doctor Who turns a ridiculous villain into a tragic figure.
The Doctor and Rose, having cured an entire hospital and given Cassandra closure, head back to the TARDIS for more adventures — Rose still slightly traumatized from the whole “possessed by a moisturized trampoline” thing.