Quite a lot of firsts with this one; first combination of historical and sci-fi (beyond the existence of the TARDIS), first Monk story, first story to feature another Time Lord, first to see anyone go into another TARDIS and the first story to be set during the Viking Invasion of England (1066 AD, ho ho).

Now, it's not as bombastic as that may make it sound, (though seeing Vicky and Steven go into the monk's TARDIS did get a bit of a gasp out of me) no, mostly it's a fun historical romp with a sci-fi twist, by modern standards it would probably be criticized for being quite dull and not using its concept well enough, and I must admit I do wish they were explored some more but it's decently fun nonetheless and I guess you could say it doesn't overburden or overwhelm, it simply introduces a few new interesting concepts and has them play out.

I guess you could say it's high in concept but low in execution, which is a bit of a shame as they have already done some high-concept stuff thus far, like Dalek Invasion of Earth and Planet of Giants, so they can definitely pull it off, though I can see how the Monk meddling in time could be quite difficult to show at the time, but I still wish they had.
For me the biggest sin of this serial is that it tells more than it shows, it tells us that The Monk has meddled in time, but it doesn't show us it happening in a way, no The Chase-style cutaway to The Monk changing any events, no plan played out that changes history, it's all just theoretical, which is a shame.

More positives: the performances are good as always if you can look past a few fumbles here and there. I'm missing Ian and Barbara, but Vivki and Steven work really well together, I love their bickering. The Monk is good fun and played very well by Peter Butterworth and the final showdown with The Doctor is quite good and quite clever.

So altogether, not the finest this era has to offer, in my opinion (though I realize I am in a bit of a minority when it comes to that), but certainly good fun and well worth a re-watch.
7/10