This ship is carrying the population of Earth, both human and animal, and a group of aliens called the Monoids. It's ten million years in the future, and Earth is about to be swallowed by an expanding sun. So they're making a 700-year journey to another planet to make a new home. But wait, Dodo brings the common cold along and almost kills them all since they have no resistance. Luckily, the Doctor finds a cure and they go on their way.
The TARDIS dematerializes and reappears in the same location. But now it's 700 years later. The Monoids are in control of the ship, all because that cold wasn't really cured by the Doctor. It mutated, and the Monoids enslaved the weakened humans. Let's make a long story short...too late. The Monoids are jerks who want to leave the humans on the ship and blow it up. Some of them don't. They fight amongst themselves, and the humans and the good Monoids live on the planet together with the current, invisible (?) inhabitants. The end. I recommend skipping this one, although I did think that ending part two with them leaving and starting part three with them materializing 700 years later in the same place was great. Too bad it didn't live up to its potential. Maybe if they had left Dodo on this planet...
Companions: Steven, Dodo
Episode: #23, "The Ark," four parts- 'The Steel Sky,' 'The Plague,' 'The Return,' 'The Bomb'
Adversary: the Monoids
Classic Lines: while the Doctor is giving Steven the treatment, Dodo, to the Doctor: "Don’t you have to squirt it into his arm?" the Doctor, in reply: "What with a hypodermic needle? Good gracious, no. That went out a long time ago."
Tuck This Away to Impress Your Friends: The treatment for the common cold devised by the Doctor was a mixture made from animal membranes.
Next Up: "The Celestial Toymaker"