The Timeless Child plot twist from the finale of Doctor Who season 12 retconned the entire continuity of the show, but we may have an idea about what happened to the Time Lord before his life as William Hartnell’s First Doctor started.
If Chris Chibnall’s tenure as the showrunner of the BBC’s widely recognized sci-fi series did at least one thing for certain, it was piss off almost half of the fandom by breaking the lore apart in the final episode of Jodie Whittaker’s second year as the Doctor. As it came to pass, we learned that the Doctor isn’t actually Gallifreyan. Instead, the indigenous people of Gallifrey found her next to a portal that led to another universe. By experimenting on her seemingly infinite regeneration cycles, Gallifreyans found a way to splice their DNA with the Doctor’s, thus granting themselves the ability to regenerate.
This meant that Jo Martin’s secret Doctor was one of these incarnations before her/his memory got completely wiped, allowing the character to live out his life normally from “An Unearthly Child.” Of course, the new twist has a lot of implications for the world of Who, but if you’re still wondering why the Time Lords wiped the Doctor’s memories, specifically after so many regenerations, then a new fan theory might help you with that.
In the classic Who story titled “Arc of Infinity,” it was revealed that the ancient and powerful Time Lord known as the Omega was trying to break out of his prison in the antimatter universe via the Doctor’s biodata. The Time Lords believed that this collision of matter and antimatter would destroy the fabric of reality. Fortunately, there had been such precedents before, and the Council had to take extreme measures to avoid the destruction of the universe, even if it meant breaking the law against capital punishment on Gallifrey.
So, what if the Time Lords decided to fake the Doctor’s execution by wiping his memories, thus breaking his bond with the Omega and preventing his return? This would certainly help shed light on the scenes that depicted the Doctor as a policeman in the last two episodes, and it’s safe to say that Chibnall will eventually get around to explaining them.
At any rate, the mystery surrounding the Timeless Child on Doctor Who seems far from over, so we’ll just have to wait and see what Chibs has planned for the next major story arc.