In the final days of Earth, the TARDIS lands on a massive spaceship carrying the last members of the human race to a new home beyond the solar system. The humans are not out of danger, however, as a deadly plague emerges from the most unlikely of sources: the common cold carried by the Doctor's companion Dodo. Put on trials for their lives, the TARDIS crew must find a way to repair the damage they have unwittingly caused - and face up to the devastating consequences.