The planet Alzarius is home to a group of people who live in a community outside the hull of their crashed Starliner. 4000 generations have passed since the ship crashed, and the Alzarians have been diligently working to repair it in hopes of returning to their planet Terradon - but although the inner workings of the ship are known down to the last detail, not a single Alzarian knows how to fly the ship. These pages were lost from the instruction manual, and apparently the knowledge was never passed down.
Life on Alzarius is generally good - most members of the community are happy living around the Starliner, and riverfruits grow in abundance. But the one blight on their existence is Mistfall. Widely assumed to be a fairy tale by those who have never lived through it, Mistfall occurs every fifty years when Alzarius' orbit takes it away from the sun. The first sign of Mistfall is, unsurprisingly, mist - it begins bubbling from the water and swathing the countryside. This is a sign for all the Alzarians to shut themselves up inside the Starliner; the Deciders who oversee the community let the people believe that the mist is toxic, because it keeps them from venturing outside during Mistfall. In actuality, the mist merely has an odd smell - the true danger lies elsewhere.
Then things begin stirring under the water. Anyone unlucky enough to be at the water's edge may be dragged under its surface by unseen hands. As Alzarian legends say, "When Mistfall comes, the Giants leave the swamp." And indeed, shortly after the mist has blanketed the landscape, figures begin to emerge from the water. These are Marshmen, brownish-orange colored creatures with bumpy skin, dark slitted eyes and tendrils hanging from their faces. The Marshmen initially take some time to get used to their new, water-free environment, but they are highly adaptable and learn to do this quickly. When enraged or frightened, they make a growling, screeching noise; young Marshmen make a similar noise but it is more of a mewling. They show highly intelligent behavior, although they are also inherently violent and will take up clubs to attack any people that they find wandering around. They can only be driven away by pure oxygen, which is too rich for their lungs to metabolize until they can adapt to it.
Each Mistfall is preceded by unfamiliar insect life, large spiders whose embryos develop inside the riverfruits, from which they eventually hatch. These spiders have glowing yellow eyes, red bodies and very long fangs. The spiders' bite transmits some kind of nontoxic psychochemical; and they can apparently only bite once, after which they die. This bite transmits an alien protein present in the spiders' brain tissue, and it establishes a psychic link between the spiders' victim and the Marshmen. Shiny veins break out on the victim's face and hands, and they begin to aid the Marshmen, who behave as if they either defer to the victim or fear them. The victim, apparently, will gradually continue changing into a Marshman unless the process is stopped. It seems that the mere touch of a Marshman has the same effect, and it affects Alzarians as well as aliens.
What the Alzarians do not realize until the Doctor's visit is that the entire life-cycle of the planet is linked. The spiders eventually evolve into Marshmen, and their bites also transform people into Marshmen. The Marshmen, adaptable enough to learn to breathe air and even read, eventually came to resemble the original crew of the Starliner. Presumably the original crew died, and the evolved Alzarians replaced them, eventually forgetting their origins as Marshmen and hiding themselves away during every Mistfall so that they never came face-to-face with their ancestors. The Marshmen's adaptability also helped to explain the Alzarians' quick healing through rapid cellular regeneration. Realizing that they did not know how to pilot the Starliner because it was not theirs to begin with, the Alzarians began to despair of ever getting to Terradon, which was not even their home planet. But the Doctor helped them to fix their ship, and showed them how to pilot it - then left the decision to stay or leave up to them. In the end, the Alzarians decided to leave their home planet...but instead of going to Terradon, they would find a new home.
