The question at the heart of “The End of the World” is, appropriately enough, “What happens next?” This not only permeates the episode and colors the actions of its characters, but also inevitably rests in the mind of the viewer who, having been reasonably pleased with “Rose,” now wants to know whether this series will continue to captivate. To answer that question, writer Russell T. Davies weaves a tale that emphasizes death, loss, and the challenges of moving on, all centered around the planned demolition of planet Earth five billion years from now.