Doctor Who is the world's longest-running science-fiction TV series, but key episodes have been lost - including William Hartnell's regeneration story. When Doctor Who began, nobody expected it to last for so many years. The series was originally envisioned as an educational program, using time travel to explore the past. It didn't take long for Doctor Who to change shape, though, with the Daleks introduced in the second adventure.
In 1966, William Hartnell - his health suffering - finally acknowledged it was time for him to move on. Usually that means a TV show will come to an end, but the BBC hit upon an audacious idea. They came up with the idea of regeneration, and Hartnell was replaced by Patrick Troughton. Regeneration became central to Doctor Who, allowing the series to replace its star time and again, with each showrunner given the opportunity to reinvent it.
William Hartnell's regeneration is a key moment in sci-fi history - but, surprisingly, the regeneration episode was lost. Back in the 1960s and 1970s, the BBC completely underestimated the potential value of Doctor Who. As a result, they "junked" innumerable episodes from the Hartnell and Troughton era, destroying old videotapes in order to make room for new ones. This included the final episode of "The Tenth Planet," William Hartnell's regeneration story. Only a few minutes were preserved - ironically because a clip was shown on the children's series Blue Peter.
253 episodes of Doctor Who were broadcast in the '60s, but initially only 156 survived in the BBC archives. Shockingly, that gives Doctor Who a better survival rate than many other contemporary programs. Since the late '70s, the BBC has conducted an intensive effort to recover lost footage, discovering missing episodes through tapes sold to international broadcasters, or occasionally on a home videotape. Thanks to fan efforts, the BBC has successfully acquired a complete audio archive of even the lost Doctor Who episodes. Some have been released by Big Finish as audio-dramas, while the BBC has animated others. This is easiest with episodes set in the present or the future, given that the historical episodes tend to be too detailed for the animators to recreate.
And so, one of the most important moments in the history of Doctor Who has been lost. The clip from Hartnell's regeneration has been used many times, incorporated into other stories as a flashback. Even today, fans scour the globe and run campaigns promising lavish rewards to anyone who finds "The Tenth Planet" episode 4. But by this point, a copy is unlikely to ever be found.