The Tardis arrives inside the garden of what turns out to be a huge spaceship, which new companion Dodo Chaplet dubs "The Ark". The Doctor, Dodo and Steven discover that they've arrived in the far future and the Earth is no more. The human race is now stored in databanks on the Ark, with the exception of a few Guardians chosen to maintain the ship and the databanks. Also on board are an alien race known as the Monoids, who are distinguised by their one eyed appearance. The Monoids are treated as indentured servants by the human Guardians, rather than as equals. The Ark's destination is the planet Refusis, a new world that the humans and Monoids intend to settle. The entire voyage to Refusis will take 700 years.
Dodo has come down with a cold, and the Monoids and the Guardians start getting sick. The Monoids and the human Guardians have never been exposed to this particular cold virus and have no immunity. The Commander of the Guardains succumbs to the cold and several Monoids and Guardians have perished due to the virus. The time travelers are condemned to death at a trial headed by Zentos, the second in command. The Commander orders that the travelers be spared from his sickbed, so that The Doctor can work on a cure. The Doctor is able to do so, and the Guardians and Monoids are cured. The Doctor and his companions take their leave, and resume their travels.
Suprisingly, the Tardis lands in the exact same location, except that the Ark has now almost reached its destination, the planet Refusis. The Monoids are now in control, having gained control during a second bout with the virus. The human Guardians now act as servants to the Monoids. The Monoids capture The Doctor and his companions. Steven is put to work in the kitchen with the Guardians. The Doctor and Dodo are sent down to Refusis, along with Monoid 2 (the Monoid second-in command) and a Guardian servant by the name of Yendom.
While down on Refusis, The Doctor meets up with the Refusians, who no longer have physical bodies. They are more than willing to share the planet with the humans and Monoids, but will only share with beings who are committed to a peaceful existence. Monoid 2 returns to the landing pod to warn the Monoids of the Refusians, but the landing pod is destroyed along with Monoid 2.
Monoid 1 (the Monoid leader) has decided that the humans will remain on the Ark, which will then be destroyed by an atomic bomb. The Guardians, finding out about this plan via Yendom, start a revolt only to find that the Monoids have left the Ark for Refusis. As the humans search for the bomb, the Monoids discover the destroyed landing pod. Convinced that the planet is dangerous, Monoid 4 leads a revolt against Monoid 1 and a battle ensues in which many Monoids are killed.
The Doctor steals one of the landing pods left by the Monoids and returns to the Ark with Dodo and a Refusian. With the Refusians help, the bomb is disposed of before it can destroy the Ark. The humans begin landing on the planet, and the Monoids who have survived the battle make peace with the Refusians and the humans.
The Ark is really two stories set in the same place but at different times. The first story is a somewhat familiar one, dealing with a killer virus. In this case, it's simply nothing more than a common cold, but the humans and Monoids have never been exposed to the virus, thus causing an epidemic. The epidemic causes widespread panic among the Guardians, which is what leads to putting The Doctor and his companions on trial. I found this first part of The Ark to be somewhat predictable, after all it was highly unlikely that The Doctor wouldn't find a cure and save the day.
The second half of the story, wherein the Monoids have turned the tables on the human Guardians I found to be a bit more interesting. The humans are divided into two factions, those who collaborate with the Monoids while the others wait for an opportunity to rebel against them. The Monoids are similarly divided between those loyal to the leader Monoid 1 and those Monoids who question Monoid 1's leadership (led by Monoid 4).
The Refusians, unfortunately, are the weak link in the story. So powerful that they can move a statue that weighs many tons with ease, one does have to wonder why they are bothering with allowing the humans and Monoids to settle on their planet. The Monoids don't seem like much of a threat to the Refusians, so it's a bit hard to generate any tension when you realize that the Refusians could defeat the Monoids quite easily.
The new companion Dodo is something of a return to the Susan/Vicki mold, a bubbly young teenage girl. After the more unusual companions (or pseudo-companions) Katarina and Sara Kingdom, Dodo is a bit of a letdown. It isn't that Dodo is that bad of a character, it's more that I've seen this kind of character before and done better.
Overall, I found The Ark to be somewhat average, and at least so far has been my least favorite story from what is proving to be a very strong third series/season.
