All companions in the 2005 series come back to save the universe. When every one including the Doctor is helpless only one can save them, the great and unique: Donna Noble. The whole of creation would have disintegrated if not for the Doctor and Donna.
I don't like the way they got rid of Donna. It was great for storyline, but I'm glad she was the one who was part time lord and got to show her unique brilliance one last time. I'm also happy everyone was friends with her, and Rose wasn't jealous or mean to her. She was unique in this series in that she was the Doctor's best friend. I also can't believe Tennant's leaving Doctor Who soon. They could bring him back though, with that other Doctor in the alternate reality with Rose. The writers left some room for ways to bring all the recent companions back into future episodes, if you think really hard and look for any way possible.
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