10 MORE Unanswered Mysteries From Doctor Who

In the ninth season of the revival, the Twelfth Doctor encounters a young Viking woman named Ashildr. She is killed by the Mire, a warrior race who have travelled through time to harvest humans. Though the Doctor puts a stop to it, he is also faced with the realisation of why he chose the face he bears. While in his Tenth incarnation, Donna Noble convinced him to save someone, anyone, wherever possible. Thus, he chooses to save Ashildr. He implants her with a device that brings her back to life.

However, life Jack Harkness, there isn't an expiry date. She is due then to live forever, never ageing. As an act of kindness, the Doctor gives her a second device. Jumping several hundred years into the future, he encounters her again. She has held on to the device, never having found someone worth sharing eternity with. Enter Sam Swift.

The likeable brigand is killed during one of Me's, as she is now calling herself, schemes that goes badly wrong. To atone for this, she revives him with the second device. He does indeed come back to life, though not even the Doctor knows what's going to happen next. Will Swift be immortal? Will the device be depleted by bringing him back from beyond death?

If not, then there is another ageless, timeless person in the universe who could well be facing the end of time and existence alongside Me, though without the aid of a TARDIS to escape.