The tracer detects the second segment on the planet Calufrax. The TARDIS makes a bumpy landing, and the Doctor and Romana soon discover that they are not on Calufrax at all. They are in fact on Zanak, a planet that has been hollowed out and fitted with engines so that it can transmat through space and materialise around others - such as Calufrax - to plunder their mineral wealt... (展开全部)