A miraculous new process has been developed that can seemingly cure anyone of their criminal tendencies. But what happens to these negative impulses once they are removed? Where do they go? Or maybe more importantly, how are they removed in the first place? This week Paddy and Tricia watch as The Doctor and Jo try to answer all these questions, unaware that the fate of the world also hangs in the balance. Can The Doctor successfully match wits with The Mind of Evil?