inside the city, the master tells the doctor that the primitives were once an advanced civilisation. before declining, they built a super-weapon that was never used and he wants to claim this weapon for himself. the room with the machinery in the city is the heart of a weapon; so powerful that the crab nebula was created during a test firing. the doctor rejects the master's overture to help him rule the galaxy using the weapon, stating that absolute power is evil and corrupting. the guardian appears, demanding an explanation for the intrusion. the master explains that he's come to restore their civilisation to its former glory. the doctor argues against him, and the guardian recalls that the weapon led his race to decay, and its radiation is ruining the planet. it instructs the doctor to activate the self-destruct, which he does. the city begins to crumble, and the guardian tells them they must leave before it is too late. while the doctor and the master flee they find caldwell and jo, and the four get out in time.