Doctor Who Series 10: 6 Theories On Who's In The Vault

Every season of Doctor Who has its central mystery. Bad Wolf. Mr Saxon. The time cracks. The Hybrid etc. Typically, some enigmatic individual or macguffin is teased throughout the run before finally being revealed in the season finale. This season the big ongoing story arc is the vault the Doctor is protecting at St Luke's University and the question of what is inside it.

Season ten is taking a different approach with this one, though. We have it on pretty good faith (or at least Steven Moffat's word...) that the truth about the vault will be revealed in the very next episode - Extremis, written by showrunner Moffat. It's a clever idea as it could help give the second half of the season a fresh spark and forward momentum.

So what do we know about whoever it is that is locked up inside the vault? The only concrete information we have is that the Doctor swore an oath to watch over them - something he has repeatedly broken this season. It has to be a pretty notable character inside there, then, right? A highly dangerous one, even? Or maybe someone who needs protecting?

From long-lost relatives to returning enemies, let's run through the biggest contenders.

Here's an out-of-the-box idea: what if the Doctor we have been watching throughout season ten so far isn't really the Doctor at all? What if the one who is currently parading around the universe with Bill and Nardole is actually an imposter who has locked up the real Doctor in the vault? You know, like Mad-Eye Moody in Harry Potter and the Goblet of Fire. Though we don't imagine that the fake Doctor is really David Tennant...

There isn't a lot of evidence for this theory, granted, but it is one possible reading of the events. The Doctor has definitely acted differently around Nardole of late. In The Return of Doctor Mysterio and The Pilot, the Doctor seemed to enjoy having Nardole around as his reliable butler but in recent episodes he has treated him with pretty much contempt. So how about if the thing in the Vault got out and switched places with the Doctor at some point between episodes one and two? That would explain the Doctor's sudden urge to go out and see the universe again.

This twist would recall 2010's The Pandorica Opens, in which another mysterious box said to have a prisoner inside was revealed to be the perfect prison for the Doctor himself. As such, it might come across as a bit cheap to repeat that revelation here. Not to mention how annoying it would be if found out we hadn't been watching the real Doctor all this time.