The TARDIS arrives millions of years in the future on a spaceship.  All the Earth's surviving life forms are on a voyage to the planet Refusis II to start anew.  Dodo has a cold which is an unknown illness to these people.  The people on the ship and the Monoids (their genetically created servants) are striken with the cold virus.  The Doctor and crew must prove that they did not spread the disease deliberately.  The Doctor finds a cure and the TARDIS leaves the spaceship.  It very soon arrives back on the ship but years in the future at the end of its voyage.  The Monoids have by this time become the dominant species and enslaved the humans.  The Doctor get help from the Refusians, who are an invisible species, to convince the Monoids and humans to so-exist on the new planet.