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.