The Doctor faces the end of his life as the Master's plans hurtle out of control. With the sound of drums growing louder, and an ancient trap closing around the Earth, the Doctor and Wilf must fight alone. But sacrifices must be made, and the deadly prophecy warns: "He will knock four times." 片一开场，就向观众呈现了一个“倒立”的世界，意图很明显，就是点题。