In answer to a summons from the Time Lords, the Doctor heads the TARDIS for Gallifrey where he must return Romana, but having tasted adventure and seen foreign worlds, she's none too happy about the return. Their journey, however, is interrupted by a rare space/time phenomena that lands them - not on Gallifrey - but on Alzarius. There, humanoid inhabitants begin to encounter an infrequent but regular cycle of perilous weather, called Mistfall, that coincides with the emergence of dangerous creatures.