hey ho, the episode is nearly over, lets spray some water about and say they are all cured. Bet those cats were annoyed they hadn't thought of that

where they ran out of time/money presumeably and cheated
How else would you save the diseased zombie people?

That part of it was about character building. It is the first episode with the 10th Doctor, you have to establish his character.

I think the hole zombie thing was in it to compare Cassandra to the Doctor. When Cassandra discovers the zombies she tries to bribe the cat people, whereas the The Doctor is outranged. Cassandra would probably try to kill them all, whereas the The Doctor saves them.