Destiny. We'd all like to think that the time of our ending is undecided until the moment comes; will you go out in a blaze of glory attacking a Dalek battlecruiser, be gunned down in one of Brigadier Letherbridge-Stewart's less successful plans, or live to a ripe old age and die in your sleep? Thankfully, most of us we won't know how it ends until it happens.
But when you step into a time machine, that doesn't hold true anymore, and suddenly you can get a sneak preview of your last days years before you endure them. Could you survive knowing that a terrible torture awaits you (not that you'd have any CHOICE!)? And more importantly, can one of our heroes?
Meanwhile, the Doctor is left wrestling with a rather more minor problem... how to prevent the destruction of an entire galaxy, armed only with a sonic screwdriver, a non-functioning TARDIS and a suicidal old bat determined to take a good chunk of the universe with her.