The episode - the first of a two-parter - is set in the 51st century on a planet housing the biggest library in the universe. So big, in fact, that the whole planet is a library. However, 100 years ago it was suddenly sealed off and its inhabitants sent out a cryptic warning: "If you want to live, count the shadows."
When the Doctor and Donna arrive on the scene, the entire library is deserted. Using one of his fancy gadgets, the Doctor determines that he and Donna are the only two humanoid life-forms on the planet, but when it comes to general life-forms, there are apparently a million million knocking around.
Naturally, the library houses its fair share of books. Being the biggest library in the universe, it has books on every subject imaginable - including detailed accounts of historic events. Events which, for Donna at least, have yet to happen. The Doctor warns her against having a peek, claiming that spoilers are best avoided. The cheek!