There have been several, especially in the early years of the classic series. The seasons then were so long (over 40 episodes) that the only way to give an actor time off for holidays was to write episodes in which his/her character didn't appear. Because the format of the show was a series of serials (multi-episode stories), this was normally done by splitting the TARDIS crew into groups & having one or two episodes that concentrated on a group that didn't include the actor's character. This meant that the Doctor appeared in every story but not in every episode. In season 1, for example, the Doctor was absent from episodes 3 & 4 of "The Keys of Marinus", to allow William Hartnell to go on holiday, his first break in seven months.
The first story in which the Doctor didn't appear was "Mission to the Unknown" in season 3. This story is an anomaly in several ways:
- None of the TARDIS crew appeared in it;
- It consisted of only a single episode;
- It was a prologue to "The Daleks' Master Plan", rather than a complete story in itself.