It is ten million years in the future and the Earth is about to plunge into the Sun. A gigantic Space Ark has been launched to take the last of humanity to a new life on the planet Refusis II. Accompanying the humans on their journey are the Monoids, strange reptilian creatures from an alien world. When the TARDIS materialises on board, the First Doctor and his friends are greeted with suspicion which soon turns to open hostility when Dodo inadvertently infects the Ark’s crew with a long-forgotten virus.