Twelve million years ago, a war touched the Earth briefly. Now, in Antarctica, an archaeological team has discovered the detritus of the conflict. And it's alive.
Twelve million years ago, a creature evolved that was capable of consuming all life in the universe. Now someone, or something, is desperate enough to want to revive it.
Outside the ordered universe, things move. They're hungry. And something has given them the scent of our space/time.
In the far future, the Doctor has learnt of the war and feels he must intervene — but it's more than just a local conflict of interest. One of the groups of combatants is from his own future, and the other has never, ever, existed.