10 Doctor Who Episodes With Disturbing Implications
6. The Metacrisis Doctor's Swift Death (Journey's End)
The Metacrisis Doctor was an identical copy of the Tenth Doctor, only with a few key differences: he aged like a normal human, only had one heart, and couldn't regenerate.
During the conclusion of Journey's End, this substitute Doctor was left on a parallel Earth with Rose Tyler, and - as revealed in a deleted scene - a chunk of TARDIS, which, given enough time, will eventually grow into a full-sized TARDIS all on its own.
This was the last time we saw Rose and the Metacrisis Doctor, but the fact that they have a TARDIS strongly hints that they'll jet off for a series of adventures in time and space, protecting their version of Earth just as the real Doctor protects his.
Sounds quite positive, but what happens next might be significantly more downbeat. Remember how we mentioned that the Metacrisis Doctor can't regenerate? Yeah, that's a recipe for disaster.
If he's travelling the universe like his prime Earth counterpart, he's going to get into a few scrapes. But when he's shot by a Dalek or a Cyberman, he won't respawn with a new face and start over - he'll just be dead. And let's face it, since this Doctor is much more reckless than the real one... that's bound to happen sooner rather than later.