This is a sort of synopsis of Necessity, with just enough information to be able to understand the next story, Incongruity.

The story so far:

"Necessity" is an alternative-universe story which alters the events of the last Ninth-Doctor story, "Parting of the Ways".

In this universe, there was no Bad Wolf. Rose got back to the Doctor the hard way. During the course of seven or so years, she studied a lot, learned a lot, and became an expert in electronics and computer security. She studied history as a hobby. You don't need to know the details, but she got back with the help of an alien whom she christened Daisy. Daisy likes Heavy Metal music. The Daleks were fought off with, amongst other things, an anti-metal virus from the Doctor's original recipe. Jack didn't die.

On to Incongruity.