The Meddling Monk
The Meddling Monk is an interesting serial, for this is the introduction of the pseudo historical. A pseudo historical is when events in time can actually be altered, changed...messed with! The Monk, played by Peter Butterworth, is quite delightful, for Butterworth plays the character with a charm that rivals that of the Doctor. In this four part story, the Doctor's edges have definitely rounded out, yet I still find the Monk interesting from a story standpoint. This is the first story where we find out that other people from the Doctor's home planet have a TARDIS. And unlike the Doctor, other people from the Doctor's home world are not as trustworthy as our major protagonist.
The Doctor displays a sense of duty, removing a key item from the Monk's TARDIS. Although some might say that the Doctor is being hypocritical, since he has taken such actions that could influence the course of time, I believe that his hearts are in the right place. Unlike the Monk, who was willing to take aim at the Viking Fleet for his own personal goals.
Yes, I believe this serial marks a fix point in time where this show decided to head in a direction where the show's actions, as far as history is concerned, could make a major tear in the fabric of time.