Doctor Who: Unbound: Auld Mortality

What if... the Doctor and Susan had never left Gallifrey? In a marble mausoleum, deep in the cisterns beneath the Capitol, Gallifrey's favourite author faces his ultimate destiny. Who is the woman who claims to be his granddaughter? Who is the sepulchral figure in robes of night? Which path should Hannibal's army take to Rome? And on a snowy mountain high in the Alps, the Doctor remembers the ultimate question: What if he and Susan had never left Gallifrey?