The half-Human Doctor was created as a result of an instantaneous biological meta-crisis. Regenerative energies present within The Doctor's severed hand combined with the Human DNA of Donna Noble when she touched the hand's casing; the force of the reaction shattered the case allowing the energy to regenerate the entire missing body of the Doctor creating a Time Lord/Human hybrid. A hybrid that possessed one heart; aged as Humans do and with no ability to regenerate after death. As a result of growing partially from Donna's DNA the new Doctor picked up parts of Donna's speech but maintained that he possessed the same memories, thoughts and feelings of the original Doctor--essentially they were the same man with physiologically differences. However, the original Doctor is quick to point out his new double was born of battle, full of blood, anger and revenge which enabled him to commit genocide against the Daleks, something the original Doctor believed himself incapable of after the events of the Last Great Time War.
Feeling his new half-Human counterpart is too dangerous to be left to his own devices, the Doctor entrusts him to Rose Tyler taking the two of them back to Bad Wolf Bay in the parallel universe dubbed Pete's World. The Doctor tells Rose that his half-Human self needs her, as he is angry and vengeful like he once was when he met Rose as the Ninth Doctor. He tells her that she made him better, and now she can do the same for his other half-Human self. She objects to this at first, insisting the new Doctor isn't really him despite the Doctor's assurance that they are the same man and the half-Human Doctor offering to spend the rest of his life with her.
However, Rose then asks both Doctors what was the last thing they said to her when they were previously standing on Bad Wolf Bay (Doomsday). The Doctor is unable to give her a direct answer only allowing himself to hint: "does it really need saying?" At this point she turns to the half-Human Doctor and poses the same question, to which he steps forward and whispers the answer his other self could not say in her ear. Executive Producer Julie Gardner confirms during the Doctor Who Confidential episode "End of an Era" that the half-Human Doctor whispers "I love you" to Rose.
Seemingly overcome with the revelation, Rose grabs the lapels of the half-Human Doctor and pulls him into a passionate kiss, which he reciprocates. The original Doctor looks on sadly for a brief moment before returning with Donna to the TARDIS and leaving the parallel world for good without saying goodbye to Rose. As the doors of the TARDIS slam shut, Rose and the half-Human Doctor break from their kiss, but it is too late. The Doctor has already left and Rose is once again stranded in the parallel universe. But this time, there is a Doctor she can live a normal human life with by her side.
Notes
The clothes that the new Doctor chooses reflect the proposed outfit Tennant was to wear in Season Three opener Smith And Jones, with the Doctor wearing a red t-shirt under his blue suit as opposed to the light blue shirt/tie combo that was eventually used. (DW: Smith and Jones (commentary))
The revelation that Time Lord/Human hybrids are incapable of regenerating and only have one heart has put to rest the long debate over the validity of the Eighth Doctor's claim of being half-human, although it is still suggested that the Doctor himself is more than a Time Lord as shown DW: Forest of the Dead when he discovers his abilities are evolving beyond what Time Lords are normally capable of.
Behind the scenes
- Although he the same appearance, memories, and basic personality of the Tenth Doctor, the new Doctor also exhibits several personality changes, in particular based upon that of Donna Noble. It is left deliberately ambiguous as to whether the new Doctor is actually a new incarnation of the original Doctor (if not strictly speaking a new regeneration) or a copy.
- In the Doctor Who Confidential installment, "End of an Era", executive producer Julie Gardner confirms that the new Doctor does indeed say "I love you" when he whispers in Rose's ear.
- In the original script the "original" doctor was ment to give a TARDIS coral to the half-human Doctor but it was later cut.
