The BBC website reports:
TARDIS is a dramatic, non-stop story in which you get to take control of the Doctor's time machine and battle a new monster known as the Entity... But if you want to know what the Doctor and Amy are up to immediately before that adventure begins, you can find out now in Wish You Were Here. It's written by James Moran (The Fires of Pompeii, Torchwood) who also wrote the latest Adventure Game. It sets the scene for TARDIS, introduces a new threat faced by the Doctor and leads directly into the action of the game.
You can download the mini-adventure (PDF format) from the BBC website, and find out all the details about the new adventure, screensavers, a 360° panorama, etc. - and of course download from tomorrow - from their TARDIS section.
Karen Gillan introduces the new Adventure Game: