As a matter of interest, this particular serial was voted by fans of the show in the UK as the best William Hartnell story – I can see why.
This was the first “First Doctor” story I ever watched, and had nothing really to compare it to, until I bought the box-set of the beginning. The early stories made no sense whatsoever, and left the audience to think “Oh well, it's Sci-Fi, I guess it's tough to understand.” (Or maybe that's just me…? Probably.)
In this serial, the major plot element is on the cover and this website – it's the first time the Doctor (and hence the viewer) meets another Time Lord (in the history of the program) who is meddling with events in time. The Doctor deals with him swiftly, and moves on.
The story is, for once, well paced, well developed, and I enjoyed it. Mainly, because it made perfect sense.