Parts: 4
Writers: Paul Erickson and Lesley Scott
Airdates: 5 March 1966 - 26 March 1966
Production Code: X

Episode Titles: The Steel Sky; The Plague; The Return; The Bomb

Doctor: first
Companions: Steven, Dodo

Creature Feature: Monoids, Refusians

Story: Dodo finds it hard to believe that the TARDIS has actually travelled in time when they land in a lush tropical jungle; to her, it looks just like Whipsnade Zoo near London where she used to go on school trips. To prove it, Dodo points out several Earth animals - but when the TARDIS is surrounded by a group of curious one-eyed lumbering creatures, Dodo can find no protests.

The creatures are called Monoids, servants of the human Guardians who control the Ark - a large spaceship carrying the last of humanity to a new home on the planet Refusis II. The Earth is dying and will soon be destroyed, so the remaining humans have packed up their things and departed to a new life.

But Dodo has come on board the Ark with a nasty cold - hardly life-threatening to her, but extremely dangerous to a race of humans whose last exposure to the cold was so long ago it has been forgotten. The residents of the Ark begin dropping like flies, and even if the TARDIS crew can win back their trust long enough to save them, the illness will have long-lasting effects...