After saving the world using fruit, the Doctor decides to really impress Rose by taking her to New Earth-in the year 5,000,000,023. So...we've lasted pretty long then. Great!
However, there are also cat people-specifically, cat nuns, which makes a hilarious sort of sense when you remember how cats were (and are) frequently found in monasteries and convents. The Sisters of Plenitude oversee a hospital, helping to cure patients of previously incurable diseases. However, the Face of Boe is the one who summoned the Doctor to the hospital. Meanwhile, Rose gets kidnapped by Lady Cassandra.
Wait, what? Yes, Lady Cassandra. She didn't quiet disintegrate, and has decided to exchange her thin body for a...err...bouncier one.
After a number of wacky shipping hijinks, the Doctor and Cassandrose discover the secret to the Sisters' cures: artificial humans grown to be used as lab rats to discover the cures. It's effective! It's also horrible!
Cassandrose of course uses this to her advantage by attempting to blackmail the Sisters, then attempting to cause a zombie apocalypse. Luckily they can lure said zombies through a disinfectant shower to cure them and WOW THAT WAS ACTUALLY PRETTY EASY.
Lady Cassandra finally pops out of Rose's body, and (after a brief stint in the Doctor's body, which led to pure unadulterated hilarity) accepts her clone-slave-person Chip's offer of his own. She then goes back in time and is nice to herself, and...there's some lesson here.
No, really, it was a sweet ending, and made Lady Cassandra more than just a two dimensional character (literally and figuratively). The quick cure of the "infected" also showed the real cruelty of the Sisters of Plenitude-they could have cured those people at any time, very easily, but they wanted to use them as lab rats.
Also, we will always have this:
Tooth and Claw
The Doctor has been chasing this wee naked girl over hill and dale, and of course he's James McCrimmon, can't you see the resemblance? After accidentally ending up in Scotland in 1879, the Doctor and Rose run into Queen Victoria, who is scandalized by the naked girl. They travel to the Torchwood Estate, one of the late Prince Albert's favorite places. Unfortunately, the Torchwood Estate has been taken over by ninja monks.
NO. THERE ARE NINJA MONKS. IT'S LIKE AN EPISODE OF AVATAR.
Flying Monks, Obscure Reasons
To make a long story short, the ninja monks want to turn Queen Victoria into a werewolf, because reasons. It turns out the estate is meant to trap the werewolf, which is actually an alien, using a diamond and the full moon. It still makes better sense than Twilight.
Queen Victoria finally declares to Rose that she is not the least bit amused, and banishes them both, because the Doctor can be a bit much for people sometimes.
For the record, the Doctor doesn't resemble Jamie in the least.
The Doctor would never wear that silly Beatles haircut that an 18th century Scotsman inexplicably liked.
IN WHICH EVERYTHING IS WONDERFUL AND AMAZING BECAUSE SARAH JANE IS BACK.
The Doctor tries to teach physics. It makes no sense, but there it is. Strange things are going on at Deffry Vale School, and while the Doctor fails at teaching in a traditional setting, Rose is a cafeteria lady. The school really loves chips, which is cooked in oil that must be handled with hazmat suits on. This in and of itself isn't that shocking, because cafeteria food.
What is shocking is when Headmaster Finch introduces the new teacher "John Smith" to the reporter, Sarah Jane Smith. The Doctor has a silent squee to match the very loud squees of the fans, and we get large helpings of feels later on when Sarah Jane (breaking in to investigate, because that's how she rolls) discovers the TARDIS.
The Doctor was completely unaware that he had left her in Aberdeen instead of Croydon.
"...made it home all right?"
At first, we get an uncomfortable amount of possessiveness from Rose regarding Sarah Jane, until they find greater fuel in laughing at the Doctor, and Mickey discovers that he is the "tin dog". Also, there are bats. Not regular bats, giant alien bats, because of course.
Oh yes, K-9 is revived! He is still less annoying than Adric.
The long and short of it is, the Krillitane are trying to solve the Skasis Paradigm, which shows us how the universe works, because that couldn't possibly lead to insanity or anything. Finchitane offers the Doctor a chance to rule together, which is ignored. K-9 blows up the chip oil and himself, but it's okay because the Doctor had a spare. (There's always a spare K-9 around.)
Sarah Jane takes her replacement K-9 off to have new adventures on her own show, thank you very much, and Mickey decides to join the TARDIS crew.