In its third year, Doctor Who was running into a few problems. Several change overs in the cast and production crew along with the ailing health of the lead actor William Hartnell threatened to accomplish what several monsters and mad scientists failed to do, destroy Doctor Who forever.

However, an inspired decision was hatched that prolonged the program's life by replacing William Hartnell with another actor playing the part of the Doctor. Previously, there was some thought of replacing Hartnell earlier in another adventure, The Savages, but that came to pass. Instead, a story was written in which the Doctor defended a polar military base from alien attack at the cost of his own life. At the conclusion of the fourth part, William Hartnell underwent a bizarre transformation and arose as a new man. Actor Patrick Troughton was cast as the second Doctor (a move that even Hartnell himself supported).

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(for my full article on The Tenth Planet, click here)

Due to the loss of many classic episodes of Doctor Who, all but a few clips of the Tenth Planet's final part exist, robbing fans of the most important moment in the program's history. All of that will change when later this year this adventure will be released in is near-entirety with an animated finale.

Via DoctorWhoTV:
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The Tenth Planet is to have its lost fourth episode animated for a new DVD release, BBC Worldwide has announced.

Commissioning editor of the classic DVD range Dan Hall spoke about the news at the Gallifrey One convention this weekend.

He said: "It's a real thrill to be bringing such an iconic Doctor Who episode back to life. Without the events established in The Tenth Planet episode 4, there would be no Doctor Who as we know it!"

The missing episode features the departure of First Doctor, William Hartnell. It has been missing from the BBC Archives since the mid-1970s, and will be reconstructed using the same Thetamation process used to recreate The Reign of Terror episodes 4 and 5.

The DVD will also feature a special making-of featurette called 'Frozen Out' that tells the full story of the arrival of the Cybermen and the first regeneration.

The exact release date is still to be confirmed, but is expected late 2013.