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 miniaturised 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. The Guardians place the travellers on trial and Steven is forced to defend them against allegations that they spread the disease deliberately. Fortunately, the Doctor finds a cure. The TARDIS leaves the spaceship, which Dodo has nicknamed the Ark, only to arrive back there as it is approaching the end of its voyage. Partly as a result of the earlier plague, the Monoids have now grown strong and enslaved the humans.