The search for the Key to Time takes the Doctor and Romana to Calufrax. The coordinates are right, but it's too warm and inviting a world to be Calufrax. In fact, the world is Zanak. But the Tracer is going berserk -- it's sensing the segment everywhere. It turns out the planet Zanak has been hollowed out and fitted with engines, and is now piloted by the demented Captain - all robotic parts and a nursemaid - who "jumps" it into position around other planets, sucks them dry and mounts the shrivelled husks in a trophy room.