Aside from the Doctor, Donna, River Song and her party, the Library appears empty; there’s not another living soul on the whole planet. It should be a comforting thought, reassuring, but it isn’t, because although their small band of humans seems to be alone, they aren’t; they’re surrounded by an invisible horde millions, possibly billions strong.
Vashta Nerada. The name means nothing to most of those present, but the Doctor knows. They’re dangerous, lethal; they live in shadows, and they look like shadows, a very effective camouflage.
The Doctor’s never known them to swarm in such numbers. And they’re hungry.