Harvest of Time

by Alastair Reynolds

The Doctor and the Master must make an uneasy alliance to overcome a common enemy, the Sild, which have been attacking the Earth and using the Master as their tool of conquest. With the Brigadier, UNIT, and others gradually forgetting who the Master is, the Doctor and the Master must defeat the Sild before the Master becomes unstitched from time and the Earth is overrun by the vicious Sild.