This was the last story filmed in the third season's production block, although it was held over until the beginning of the fourth season. During filming, the production team realized that William Hartnell's health had deteriorated beyond the point where he could continue to work. Many months' discussion about replacing Hartnell finally came to a head, and Innes Lloyd decided not to renew Hartnell's contract. It is unclear if Hartnell was contractually obliged to appear in "The Tenth Planet" or if he agreed to do so after being informed of Lloyd's decision.