“How would you feel if someone did it to you?”
Bill Potts pleaded with the Doctor to allow her to retain her memories. The Doctor relented, as Clara’s theme played in the background. It’s implied in that scene from “The Pilot” that the Doctor did remember Clara and what she had done to him. I believe that he didn’t simply remember scraps; he remembered all of their time together. He remembered, because Clara had returned and given the Doctor his memories back. With that, Clara also extracted a promise from him to guard the contents of the vault.
My theory begs one question: Why couldn’t Clara guard the contents of the vault herself? After all, she is technically immortal with a Tardis of her own. She should have the time and resources at this point to do so. I’m assuming that Clara and Ashildr have parted company, as they wouldn’t be able to get Maisie Williams back. A lone Clara couldn’t guard the contents of the vault because of an unforeseen problem.
After Clara fractured as The Impossible Girl in his time stream, the Doctor thought he’d stabilized her. He thought he’d stopped the fracturing, which had continued, even after he carried her safely out. The Doctor had stabilized Clara, preventing her from dying countless deaths. Ironically, his efforts to prevent her final death in “Hell Bent” caused Clara to fracture once more. She would meet her end regardless of whether she returned to the Time Lords or not.
Meanwhile, Clara had stymied Missy’s plans and needed to keep her in check. Clara’s countermove involved the vault. She couldn’t go to UNIT, as it would be the first place Missy would look. She needed an alternate hiding place and the Doctor.
A fragment of fracturing Clara contacted the First Doctor, who recognized her from their initial encounter. Worn out, preparing to regenerate, and surrounded by Mondasian Cybermen, he agreed to telepathically communicate with his future self. One did contact Twelve, but that process set a series of events in motion that will cause the Mondasian Cybermen to become Twelve’s problem.
I’ll be discussing the actual contents of the vault after I gather more evidence. Please let me know what you think of my working theory and share yours in the comments. Thanks for reading.