The plot is straightforward: A historical setting (1066, prelude to the Battle of Hastings.) Vikings, Saxons, and bits of technological incongruities/anachronisms and a big mystery surrounding a devious monk.
In premise, this is an exceptional story, especially for 1965. Not only is it the first 'pseudo-historical', it finally pushes "Doctor Who" into doing more than using the TARDIS just to get everybody to a funky planet where they get scared out of their wits by some plastic (or invisible!) monster, and acknowledges that there are others like the Doctor, but who aren't as moral as he is.
The Monk is a delightful character (though I disagree he is an early incarnation of the Master. The Monk clearly has a history of playing with history on a small scale for his own personal gain, the Master has a penchant for seizing power and control whereever and whenever he can.) and well played by Peter Butterworth. Admittedly, it's great fun to watch him manipulate everybody he deals with.
The only problem is that it's slowly paced. The big revelation doesn't come until the end of episode 3. Which is fine, except we're only given small hints at meddling throughout the prior ~65 minutes and everything else happens at a leisurely pace. For first time viewers in 1965, this story is superlative and makes a top-10 story. For repeated viewings or in our supposedly enlightened 21st century, the pace is somewhat slowed. It's still worthy of the top 10 designation, the ideas presented more than make up for the slowness of the plot.
Edith the monk also gets assaulted and almost raped by a Viking. For a 1965 childrens' show, this is strong stuff. (as was the attempted rape of Barbara in 1964's "The Keys of Marinus".)
A pity the source material isn't that good, but that's the BBC's fault for junking the story in the first place. At least it exists and was returned to them so we can all enjoy it.
Definitely worth the rent, at the very least.