Doctor Who Series 10: 6 Theories On Who's In The Vault
4. River Song
All right, let's say it's not the Doctor who is in the vault. But what if it's his wife instead: the one and only Professor River Song?
This wouldn't be the first time that River has been locked up, for one, as she previously served time in the Stormcage space prison for killing the Doctor. Likewise, the Doctor then helped her regularly escape - something that recalls his playful relationship with whoever is in the vault.
There is evidence to support this idea, as well, as the Doctor's wife has been referenced a couple of times in season ten so far. First, her portrait could be seen on the Doctor's desk at the university in The Pilot. Then in Thin Ice, the Doctor took Bill to the 1814 Thames Frost Fair - a location that he previously visited with River, as explained in A Good Man Goes To War. Lastly, the trailer for Extremis revealed that River's iconic TARDIS-shaped diary will make an appearance.
On the other hand, River's story does seem to be over, seeing as 2015's The Husbands of River Song perfectly wrapped up her complex timeline in a bow. But, if Moffat is feeling nostalgic and wants to bring back one of his biggest contributions to the show for his final run, it could happen.