After sacrificing himself so that Peri could live in the last story, the Caves of Androzani.. the Doctor has regenerated into his sixth form "and not a moment too soon". The Dr finds his regeneration unstable, there are problems with control. After all a time lord who is hundreds of years old must have some pent up hostility, I know I do and I'm only 50! He finds himself some garish clothing to suit his frenetic personality then tries to kill his companion by strangling her before catching a glimpse of himself in a mirror and coming to his senses.