The TARDIS arrives some ten million years in the future on a giant spaceship carrying all the Earth's surviving plant, animal and human life (much of it miniaturized and in suspended animation) on a 700 year voyage to a new home on the planet Refusis II. Dodo is suffering from a cold, an unknown affliction in this era, and as the human Guardians of the ship and their servant race the Monoids have no resistance, a plague breaks out.