Complex sentence: The Doctor is travelling alone and already in his ninth incarnation as the story begins, although his remarks about his appearance as he looks in the mirror in Rose's flat imply that the regeneration was a very recent one.
Split into simpler sentences: The Doctor is travelling alone and already in his ninth incarnation as the story begins. His remarks about his appearance as he looks in the mirror in Rose's flat imply that the regeneration was a very recent one.