The people of Belannia II see their sun, Bel, shrouded in night for a month following an impossible triple eclipse.  When Bel is returned to them a younger, brighter, hotter star, it is the beginning of the end for the entire solar system...

100,000 years later, the Doctor and Sam arrive on Belannia IV, where the population is under threat as disaster looms - immense gravitational and dimensional disturbances are surging through this area of space.

While the time travellers attempt to help the survivors and ease the devastation, a religious suicide-cult leader is determined to spread a new religion through Bel's system - and his word may prove even more dangerous that the terrible forces brought into being by the catastrophic changes in the sun...