Clara isn't the impossible girl, she is entirely possible. The Doctor is lured to Tenzalore, which is where his grave is located. The Great Intelligence kidnaps and threatens his friends (the peeps from the christmas special) in order to get the doctor to open the tomb (which is the tardis) The doctor doesn't relent, but River does, she was there when the doctor revealed his true name. The doors open, everyone walks in to see a web of white light. The white light is the doctor, as a body is... Cliche. It is a web of all of the doctor's timelines, everytime he transported in time and space a new strand was created. The Great Intelligence enters into the web and begins to kill the doctor, over a million timelines, and reverting any victory the doctor made, which also kills the Great Intelligence (he has to die in every timeline at some point in time. The universe begins to disappear and the doctor begins to die. To save him, Clara jumps into the Doctor's timeline and saves him over a million times (The dalek asylum, the christmas special, all times Clara saves the doctor) but she too has to die in the process, and dies over a million times. The doctor is saved, time reverted, Clara is dead, but not entirely. The Doctor enters into his own timeline to save her. The Clara the doctor meets in present time is the genuine article, the original. Every Clara we have met have been her, but an offshoot of the original.
