Romana has received a summons from Gallifrey, and no summons can be disobeyed. During the journey the TARDIS hits some turbulence and ends up in the wrong Universe. When the TARDIS lands at co-ordinates 10,0,11,00 by 02 the picture on the scanner is of Gallifrey but the location is not, it is the planet Alzarius.
There are two settlements on the planet: One is a group of people living on the stranded Starliner, and the second is a group of rebels. It is the time of the mistfall, and everyone is ordered inside the Starliner. During the mistfall the air is said to be toxic and it is forbidden to go outside.
During the mistfall the Marshmen emerge from the lake, and spiders emerge from fruit. The Doctor finds that there is a startling connection, and evolution on the planet has turned full circle: The spiders turn into the Marshmen, and the Marshmen destroy the Terradonians, then evolve into a copy of the Terradonians. This happens every fifty years.
The Doctor also finds that the people are not repairing the ship - they are merely replacing good components with new components. The ship has been ready for centuries. The library on board held enough information to allow them to take the ship apart, but could not tell them how to fly the craft.
The Doctor manages to get the ship working and ready to fly. The ship leaves Alzaruis en route to Terradon, breaking the full circle.
