Almost three years to the day before Doctor Who got its big revival series starring Christopher Eccleston and Billie Piper, DreamWorks Pictures released an ambitious science-fictionmovie that pre-empted a major time-travel concept from the show. It wasn’t until 2008’s “The Fires of Pompeii,” when the 10th Doctor (David Tennant) took Donna Noble (Catherine Tate) to witness the fall of Pompeii, that the show crystallized one of its most important cosmic principles. The Doctor and his companions alter history countless times throughout the series, yet “fixed points in time” are events that must always happen. If they don’t, reality itself could rip apart.