The Doctor Who episode “The Time Meddler” concluded the second season in the show’s history, and it represented another major first: in it, the Doctor and his companions encountered another time traveler, who it is eventually discovered has a TARDIS of his own and is from the Doctor’s own world. Apart from the Doctor and his granddaughter Susan, we had never met any others of the Doctor’s people. When the Doctor looks at the TARDIS of the so-called “meddling monk” (we never learn his name), he recognizes it as a “Mark IV” and thus from about 60 years later in history than his own.
This episode brought to the fore the puzzles and paradoxes of time travel in a way that no other episode had before. While the Doctor had emphasized that he and his companions “cannot” meddle with the course of history, it was not clear whether it was impossible, illegal, or merely risky. In “The Time Meddler” the the monk’s plan is to use an atomic weapon to destroy the Viking navy in the 11th century, allowing King Harold free then to defeat William the Conqueror (who presumably might not have been called that, had the monk been successful). Vicki and Stephen ponder what would happen if he were successful – presumably all subsequent history would change, as would the history books, and even their own memories of having learned the history a different way.
The Doctor mentions not changing the course of history as the “first rule” of time travel, and the monk replies by asking “And who made that rule?” The question is not answered in the episode. The monk’s aim is to improve history, and in his diary it turns out that he had discussed the possibility of flying machines with Leonardo Da Vinci. The monk himself also claims to have helped with the building of Stonehenge. In the end, the Doctor leaves him marooned, much as later in the show’s history the time lords would leave the Doctor marooned.
There is some direct religious imagery in the episode, including most obviously other time lord’s disguise as a monk and his inhabiting an abandoned monastery. (The monk at one point lends the Doctor a monk’s cloak and says it suits him). The local Saxons use the saluation “God be with you.” But the whole notion of the possibility of more advanced beings with time travel capability meddling in human history, dressed in one instance in religious garb, nicely highlights that the story itself can be viewed as having religious overtones. Later episodes would address the subject of the Gallifreyans, including but not limited to the Doctor and his traveling companions, being viewed as gods by others (see for instance “The Myth Makers” and “The Face of Evil”).