The people of Bellania 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 Bellania IV, where the population is under threat as disaster looms — immense gravitational and dimensional disturbances are surging through this area of space.