of Otherness is achieved by Pedler's focus on the theme of "dehumanising medicine" by presenting a race of humans who have replaced most of their flesh and organs with cybernetic parts. Decker also observes that The Tenth Planet plot is based on the "base under siege" scenario, a popular science-fiction device that has been reused in many subsequent Doctor Who stories, and that this serves as a metaphor for evil. Graham Sleight notes that The Tenth Planet was produced at a time when modern medicine was pioneering transplant surgery, lending a sense of topicality to Davis and Pedler's concept for malevolent cyborgs. He also finds contemporary significance with the 1960s rocket programmes, and notes that the multinational makeup of the Antarctic base crew is particularly noteworthy, having no precedent in earlier Doctor Who stories. However, he is disappointed by the overall execution of The Tenth Planet serial, finding the Cybermen "dull, stereotyped villains" and the portrayal of the Antarctic base staff dependent on "national stereotypes". The introduction of the concept of regeneration in The Tenth Planet is noted as a landmark in the show's history, and it has been credited with establishing the longevity of the television series by ensuring the survival of the character of The Doctor. Accounts differ as to the reason for Hartnell's departure from the programme; the actor's poor health is often cited, while other claims state that he was dissatisfied with the increasingly "adult" nature of the programme's scripts. Regardless of Hartnell's reasons to quit, Muir notes that while Hartnell's departure initially created a serious problem for the production team, they took the opportunity to create "an elegant, inspired solution to a casting problem" that has endured in the programme's folklore. Commercial releases In print A novelisation of this serial, written by Gerry Davis, was published by Target Books in February 1976. It was the first Hartnell-era serial novelisation to be commissioned by Target, and the first new adaptation of a Hartnell adventure to be published in nearly ten years. The novelisation largely follows the original script, but places the action in the year 2000 rather than 1986, as well as restoring the Doctor to the third episode. Also, in the first scene in which the Doctor, Ben and Polly appear (in the TARDIS), the Doctor is beginning to show signs of his failing health; sometimes mistakenly addressing Ben and Polly as "Ian" and "Barbara",