I wasn't expecting much from this story, my hazy memories of that first viewing being of a story with too many episodes for too little plot and endless scenes of plotting Sensorites. My view hasn't really changed upon re-watching for the marathon.
The Sensorites should be good, because the aliens in it are the most convincing looking yet and their cuture is developed to a greater extent than most Doctor Who monsters, certainly to the extent of the Daleks (at this point) if not more so. They're depicted as an actual alien race and not an invading force of monsters which usually indicates more thought has gone into them. Susan (shock!) is given some proper character development and the Doctor takes the clear lead for the first time.
So what weng wrong? For starters, it's not very exciting, a cardinal sin for Doctor Who - actively bad is quite often as entertaining as a classic for all the wrong reasons. It seems worse when the show is dull, and I really don't want watching these episodes to feel like a chore, but while it didn't quite sink to those depths, it was the first time I've thought about maybe skipping to the end. The plot was slow-moving enough that I wouldn't have missed much.
Okay, starting from the beginning. The TARDISeers reminisce about their adventures so far, which was a good moment to take stock of the things that have happened to them so far, establishing the dynamic now as Doctor-Susan-Ian-Barbara and not Doctor-Susan and Ian-Barbara anymore. The TARDIS takes them to a spaceship with a dead crew that turns out not to be so dead. The first two episodes are a (very) slow unveiling of the Sensorites, with plenty of corridor scenes mixed in for good measure. These episodes are often cited as the best of the story, and they have their moments - Susan taking things in her stride is refreshing, almost making me wonder whether her reputation as the show's biggest screamer is down to Terry Nation's stories. We also get some hints and details about humanity's future for the first time as the setting is the 28th century, and I liked getting some detail to the Doctor Who universe, but there's no reason both of these episodes couldn't have been combined into one. They're not eerie or creepy, as intended, they're just slow, in fact at the start of the second episode when Ian notices that Maitland and Carol have been rendered immobile I didn't clock it, I thought their reactions were supposed to be prolonged to pad out the episode.
So I disagree with the consensus, I think these episodes are the low point of the story. The Sensorites do impress, at least visually. Their telepathic abilities are interesting and are utilised well throughout the story. I did wince a bit when one of the Sensorites admitted to being scared of the humans to another Sensorite when standing right in front of the humans. It was probably done to help make the Sensorites people instead of monsters, but I found it a very strange line.
The next phase of the story takes place on the Sense Sphere (cool planet name) and we're introduced to various Sensorites identifiable by title rather than name, as the Doctor volunteers to try to create an antidote to a disease which has been plaguing the Sensorites, in exchange for the TARDIS lock which the Sensorites have stolen. The next few episodes sees the City Administrator plot against the TARDISeers and try to stop them from helping the First Elder. The dramatic thrust of the story changes from the mystery of the Sensorites to the Administrator's machinations when he realises he can post as the Second Elder as only their sashes distinguishes them from each other. Funny thing to occur to a member of an advanced space-faring race as if it's a genius idea.
Though this finally gives the story some feeling that it's going somewhere and has purpose other than getting back to the TARDIS, this plotline isn't so good, partly because the Administrator's goals are inconsistent, with him sometimes plotting against the Doctor and his companions and sometimes plotting against the First Elder himself. The middle episodes are quite dialogue-heavy, and although at first I'm interested in the Sensorites as I learn more about their race and society, they're a boring lot and quickly wear out out their welcome. Above all, I don't know why The Sensorites is a six-parter, it has a clear four-episode plot: Episode 1 - Spaceship, Episode 2 - Sensorite city, Episode 3 - Villain plotting, Episode 4 - Surviving humans. The main thing it needed less of was the City Administrator's scenes, considering his plans involve destroying the Doctor's cure that was to be given to Ian, so that he could prove there was nothing wrong with Ian, but then somebody else gives the cure to him and the last couple of scenes are rendered irrelevant and not mentioned again. Unfortunately this is not the last pointless plot resolution in the story.
As the story gets closer to its conclusion, the Doctor investigates an aqueduct where the water has been poisoned. It's dark but not scary, but this could be due to the direction, which so far has been uninspired. I enjoyed the Doctor getting a chance to test his scientific skills, although in the Doctor's lab scenes I couldn't help picturing Pertwee's Doctor in his place. It's been obvious since the Sensorites mentioned not all humans were accounted for that they were poisoning the water, but the last couple of episodes are nevertheless quite interesting. There are more story strands - the Doctor and Ian with the human survivors, Susan and Barbara aiding the First Elder, and the City Administrator (now Second Elder) continuing to plot, though quite what he's plotting is anyone's guess considering he's achieved his original goals. The lack of clarity in the objectives of the Administrator hurts the story at this point because it leaves me unsure of where the story is going, and causes it to lose that sense of forward motion. Five episodes in, it doesn't feel like it's reaching a conclusion, it's just more things happening.
The ending is a bit rubbish. The human survivors don't have anything interesting to say and are caught pretty easily, and the City Administrator is apprehended off-screen after four episodes of buildup - when it finished, I had to rewind it as his arrest is referenced only briefly and I missed it the first time. If it's such a pointless plotline, why dedicate so much screentime to it? By relegating it to padding, the writer (wonder why they name asked him back? lol) deprives the story of what could have been a conclusion that redeemed it. True, the Doctor and his companions get their TARDIS lock back, but save for some interesting aspects of the Sensorites themselves, little about this story has been satisfying. All the plot point may have been wrapped up, but they've been done so in such a manner that I'm left wondering what the pont in them was in the first place. This wasn't an action-adventure sci-fi story, more an ideas show, and a mystery, in fact I'm not sure what it was supposed to be, perhaps that's the problem. Don't get me wrong, I'm glad I watched it, but it's a story that would benefit from being condensed to four episodes, given a better script with a better story structure and improved direction.
Horror quotient - It's perhaps the first Doctor Who that's supposed to be scary. The appearance of the Sensorites is odd, but if we invest in them as characters they're not scary.
Comedy quotient - Hmmm....
Drama quotient - For the TARDISeers, this was a simple story. Separated from the TARDIS, lets get back. Surprisingly, Susan gets the best stuff when she stands up to the Doctor.
Good set design, but the least entertaining story yet.