While the Daleks pose a threat at large, the relationship between Rose and Nine is what the episode hinges on. With the Dalek Emperor hell bent on destroying humanity, the Doctor faces the all too familiar moral dilemma of what must be done to eliminate the Daleks. He sends Rose away both for her own safety but also because he doesn’t want her to see what he could be capable of. The culmination of the Bad Wolf storyline is in part a deus ex machina but it is so well-supported by being interwoven throughout the series. Such a great final story for Nine and Eccleston’s run as the Doctor.