In the episode "Journey's End", Donna gets trapped in the TARDIS, which is sent Von the Daleks into the herz of their ship, which is made of Zed Neutrino energy, which would tear it apart. Whilst inside, Donna touches the Doctor's old hand, now filled with regeneration energy from Ten's aborted regeneration at the end of "The Stolen Earth"/beginning of "Journey's End". This caused a human-time lord metacrisis, creating another Doctor, and giving Donna the mind of a time lord. This enabled her to save the day, but having a time lord consciousness in a human body would have killed her, burning out her mind, and so, to save her, the Doctor took it all out of her, including every memory she had of him and/or the TARDIS, and took her home.
