In "Journey's End" the Doctor regenerates just enough to heal himself, moving the rest of his 'Time Lord' regenerative energy into his hand that was cut off in the series two episode "The Christmas Invasion". Donna later touches the hand, and the energy that flows into her creates a weird event called a 'biological metacrisis'. This metacrisis causes the hand to develop into a part human, part time lord version of the Doctor, complete with the Doctor's memories, personality and knowledge, but exhibiting traits of Donna's personality. This mixed version of the Doctor also has only one heart, and he can't regenerate. The original Doctor returns Rose to her alternate universe's Bad Wolf Bay. Between themselves, the original Doctor and the half-human Doctor have decided that the half-human Doctor will stay with Rose; this Doctor can give Rose a romantic relationship, as he will age and die as a human rather than greatly outliving her like his Time Lord counterpart would. The Doctor tells Rose that his half-human self needs her as he was "born of blood and fire and revenge" much like his previous body was after the Time War. He believes Rose can help turn him into a "better man". When the half-human Doctor seemingly completes the sentence the original Doctor started at the end of "Doomsday" they kiss each other. And although we do not hear what the half-human Doctor had whispered to Rose, executive producer Julie Gardner confirmed in Doctor Who Confidential that "even though we don't hear it, of course he's saying 'I love you'."