Between a 3 and a 4 for me. A start of a new season for TV, and the start to the Key to Time arc (and really, if they consider Trial of a Time Lord one long story, with 14 episodes, then I think this arc could be as tightly connected as well, otherwise I still think Dalek's Masterplan is the longest actual story).
Quite a few changes marked here, we have Romana I enter the TARDIS, with K9 Mark 2, and the tone of the show has a definitive change, where we have moved away from the horror and generally darker tones of the prior seasons, to a lighter, more humorous tone, both of which though maintain the viewer / readers interest quite well, showing one of the strengths of Doctor Who, in the variety of tales it can tell.
An interesting start here, with the idea of the Guardians, and dealing with this mysterious Key to Time, and the first adventure dealing with it is an interesting tale, with some interesting one off characters, especially Garron and Unstaffe, but the Graff as well isn't a one dimensional villain as can often be the case.
The Doctor in good form as usual, with a slightly different style given above tone changes, and Romana a good, strong willed new companion here, on an intellectual level with the Doctor, though somewhat more naïve about the universe outside Gallifrey.
K9 seems a bit more of an after thought though, and to begin with seemed to more or less disappear from the tale, to the point I forgot about him until he was brought back, which is a bit of a shame.
Unusually for an Ian Marter tale though, I found this a bit dry at times, and his tendency to elaborate a lot on the TV stories, normally a strength, just seems to add a bit of drag to this tale, without that I think would have been a solid 4.