Once again, it seems the Doctor has made a promise he can't keep, but who did he make this unkeepable promise to? The Season 10 premiere of Doctor Who introduces a new mystery when it reveals the Doctor has spent the last 50 years (at least) teaching at a university in order to protect... something. By the end of the episode, every Doctor Who fan in the audience had to be asking themselves, what is the Doctor guarding in that vault?
Showrunner Steven Moffat loves his season-long mysteries, and it seems he couldn't resist concocting one more before his time at the helm of the TARDIS comes to an end. (He's exiting the show after this season.) The premiere doesn't give up any concrete answers about what's in the vault. Although, given the imminent return of both Missy and John Simm's version of The Master, it would make sense if the Doctor has been guarding a version of his fellow Time Lord for decades.
Having two Masters running around is neither ideal or safe for the time stream. Perhaps Simm's Master is in the vault for safe-keeping, while Missy is free. That would raise a whole new set of questions about how two versions of the same Time Lord could co-exist at the same time.
The mystery isn't just what is in the vault though, it is also, why is the vault located beneath the university? I wouldn't be too surprised to discover the Doctor chose the location himself, given he is uniquely qualified to be a professor, but his insistence that he made a promise he has no intention of breaking suggests a much larger connection.
Throughout the episode, pictures of River Song and his granddaughter, Susan, from the classic Doctor Who series were shown. Both of these women are extremely important to the Doctor, and it is possible whatever is in that vault is somehow tied to them. While Bill has reawakened the Doctor's need to travel, by the episode's end he seemed torn between a sense of duty and his natural love of showing his companions around time and space.
Leaving the vault unguarded doesn't seem wise though, and since this is also Peter Capaldi's last season, his choice to show Bill the universe may result in the Doctor's next regeneration.
One thing is certain, whatever lies behind those locked doors is powerful, and the Doctor is afraid other creatures in time and space could be coming for it. And with the Doctor allowing himself to leave Earth again, anything could happen. Buckle up, Whovians, because Moffat doesn't plan on going out without a timey-wimey, mysterious bang in Season 10.