Doctor Who: Cat's Cradle - Time's Crucible (Virgin New Adventures, #5)

'You’re on your own, Ace.'
          The TARDIS is invaded by an alien presence, and is then destroyed. The Doctor disappears. Ace, lost and alone, finds herself in a bizarre deserted city ruled by the tyrannical, leech-like monster known as the Process.
          Lost voyagers drawn forward from Ancient Gallifrey perform obsessive rituals in the ruins. The strands of time are tangled in a cat’s cradle of dimensions. Only the Doctor can challenge the rule of the Process and restore the stolen Future.

But the Doctor was destroyed long ago, before Time began.