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This story is significant for two reasons. Firstly, the Cybermen, the second most popular monsters in the series’ history, make their debut. Secondly, this is William Hartnell’s final story and paved the way for the show’s future as Hartnell regenerated (although that term was not then used) into Patrick Troughton and ensured Doctor Who’s longevity. Hartnell fell ill during the recording of The Tenth Planet and so his appearances are restricted. However, when he is on screen, you can see him at his authoritative best.