Rose defeated the Dalek Emperor but she had absorbed the time vortex. The Doctor removed it from her body and expelled it back into the TARDIS. But his own body was already damaged by it. Cellular breakdown had begun. He had just enough time to tell Rose that she was fantastic and that he regretted not being able to see her again with that face before his body seemed to explode in bright, fiery light and when it was over a new man was standing in front of an astonished Rose.

Cells Breaking Down

Trying to Hold In The Pain

It Begins

The Tenth Doctor Takes His First Breath