At the end of the fourth series (the Doctor with Donna), Dalek Caan travels in time from 1920s Manhattan to the hottest battlefield of the Time War in search of Davros. Along the way, though, Caan has an epiphany. When Davros captures the Doctor and his companions, he is thwarted at last minute - Caan, who has become an oracle, reveals that he had deliberately spoken a number of false prophecies in order to lead Davros to his own undoing. Caan then reveals why he would betray the Daleks and aid the Doctor: when he made his emergency temporal shift, he fell through the Time Vortex. Just seeing the Vortex drives a significant minority of witnesses to insanity; Caan not only saw it but traveled through it for an undefined period of personally-experienced time.

But what does madness mean to a Dalek, created to kill and indoctrinated with an unshakeable belief that all other life forms must die? Perhaps it takes the form of a Dalek considering that, perhaps, other forms of life have a right to exist in peace...