A smart sequel to Rose and the Doctor's first ever off world story that, whilst fun, has the odd technical issue and storyline flaw that stops the episode reaching the top tier.
The Doctor (David Tennant) and Rose (Billie Piper) travel to New Earth, a colony set up on a planet whose orbit and atmosphere are aligned with the previous Earth, prior to the destruction we saw in the episode "The End of the World". They are called to a hospital on the outskirts of New New York, where amongst some miraculous and ahead of schedule medical breakthroughs, the Face of Boe appears to be approaching the end. But another former acquaintance, the villainess Cassandra (Zoe Wanamaker) is hiding in the basement, and on seeing Rose again, formulates a plan.
There is some bad CGI work to kick off this episode, that shows things haven't really improved that much yet, despite what must have been a higher effects budget than before. Cassandra herself, in her first form, and the transition effects, later in the episode, as people succumb to the disease are also pretty bad. The practical effects though are much better, with the cat nurses looking particularly good, along with the infecteds general boils and sores. It was interesting to read about the weather issues that affected the episode in the trivia section, as that helps explain a few times when sets are reused even when that doesn't make a lot of sense for the episode - this is particularly evident in the Zombie-esque finale.
But what the episode does do well is humour. The conceit of Cassandra jumping into other people gives, particularly Billie Piper, a chance to show some more of her comedic ability - she does this better than either Tennant or Sean Gallagher, who both just do generic posh women. There are also a couple of gags based on not quite swear words that were good too.
But ultimately those moments can't fully make up for the questionable science that makes up all of the episodes conclusion.