Alzarius, located on the exo-space universe, is an early Earth-like world, full of dense forests and large lakes and marsh land.
Every fifty years Mistfall occurs when another planet takes Alzarius away from its sun. Mistfall causes the main inhabitants of the planet, the Marshmen, to emerge from the marhland. At the same time large melon-like fruit grow from which vicious spiders hatch.
It was on this planet that a star liner from Terradon crashed. The original occupants were killed by the native Marshmen who evolved into Terradonian-like life-forms.
The fourth Doctor, Romana and K-9 arrived on Alzarius during one of the Mistfalls and discovered some alarming facts about the native Marshmen.
- Exo Space