3. The Final Episode Of The Tenth Planet (First Doctor, Season 4 - Story 29)
This missing episode is probably the most tantalizing, because so many things are happening that are of massive importance to the show's history and fandom. The Tenth Planet introduces the Cybermen, humanoid killer robots from Mondas (the namesake tenth planet in the solar system). Since this serial, Cybermen have featured prominently in the series, appearing on the show during every Doctor's run except for the Third Doctor, Jon Pertwee, and the Eighth Doctor, Paul McGann (whose only televised appearance as the Doctor was the 1996 movie). Perhaps even more importantly, The Tenth Planet
features the first regeneration in the show's history at the end of its fourth episode. In this serial, a previously-unknown planet called Mondas comes within range of Earth, bringing with it the metal terrors known as Cybermen. It is discovered that Mondas is Earth's sister planet, and that a species of beings similar to humans inhabited the planet around the same time as we were in our earliest stages of evolution. As Mondas drifted away from earth and the Sun, the Mondasians began adapting mechanical parts to their bodies to survive. Eventually, they replaced everything with machinery - including emotions. Now that Mondas is close to Earth again, the Cybermen see their opportunity to take the planet and continue to survive. The Doctor outsmarts the Cybermen and fends off the invasion, but not before falling mysteriously - and fatally - ill. At the end of the story, an unconscious Doctor's cells take on a new form, and he regenerates into the (uncredited) Second Doctor, played by Patrick Troughton. These final moments of William Hartnell's run would be iconic, if not for the fact that it's missing (along with the rest of the final episode). Luckily, the next story's first episode features a recap of the events of Tenth Planet, including the regeneration.